Current code that handles Tx buffer desciprtor ring scanning employs the
following algorithm:

	1. Restore current buffer descriptor pointer from TBPTRn

	2. Process current descriptor

	3. If current descriptor has BD_WRAP flag set set current
	   descriptor pointer to start of the descriptor ring

	4. If current descriptor points to start of the ring exit the
	   loop, otherwise increment current descriptor pointer and go
	   to #2

	5. Store current descriptor in TBPTRn

The way the code is implemented results in buffer descriptor ring being
scanned starting at offset/descriptor #0. While covering 99% of the
cases, this algorithm becomes problematic for a number of edge cases.

Consider the following scenario: guest OS driver initializes descriptor
ring to N individual descriptors and starts sending data out. Depending
on the volume of traffic and probably guest OS driver implementation it
is possible that an edge case where a packet, spread across 2
descriptors is placed in descriptors N - 1 and 0 in that order(it is
easy to imagine similar examples involving more than 2 descriptors).

What happens then is aforementioned algorithm starts at descriptor 0,
sees a descriptor marked as BD_LAST, which it happily sends out as a
separate packet(very much malformed at this point) then the iteration
continues and the first part of the original packet is tacked to the
next transmission which ends up being bogus as well.

This behvaiour can be pretty reliably observed when scp'ing data from a
guest OS via TAP interface for files larger than 160K (every time for
700K+).

This patch changes the scanning algorithm to do the following:

	1. Restore "current" buffer descriptor pointer from
	   TBPTRn

	2. If "current" descriptor does not have BD_TX_READY set, goto #6

	3. Process current descriptor

	4. If "current" descriptor has BD_WRAP flag set "current"
	   descriptor pointer to start of the descriptor ring otherwise
	   set increment "current" by the size of one descriptor

	5. Goto #1

	6. Save "current" buffer descriptor in TBPTRn

This way we preserve the information about which descriptor was
processed last and always start where we left off avoiding the original
problem. On top of that, judging by the following excerpt from
MPC8548ERM (p. 14-48):

"... When the end of the TxBD ring is reached, eTSEC initializes TBPTRn
to the value in the corresponding TBASEn. The TBPTR register is
internally written by the eTSEC’s DMA controller during
transmission. The pointer increments by eight (bytes) each time a
descriptor is closed successfully by the eTSEC..."

revised algorithm might also a more correct way of emulating this aspect
of eTSEC peripheral.

Cc: Alexander Graf <agraf@suse.de>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: qemu-devel@nongnu.org
Signed-off-by: NAndrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>