When the PTP is enabled in hardware bit 6 of PTP_MSG_CONF1 register, the
transmit frame needs additional 4 bytes before the tail tag. It is
needed for all the transmission packets irrespective of PTP packets or
not.
The 4-byte timestamp field is 0 for frames other than Pdelay_Resp. For
the one-step Pdelay_Resp, the switch needs the receive timestamp of the
Pdelay_Req message so that it can put the turnaround time in the
correction field.
Since PTP has to be enabled for both Transmission and reception
timestamping, driver needs to track of the tx and rx setting of the all
the user ports in the switch.
Two flags hw_tx_en and hw_rx_en are added in ksz_port to track the
timestampping setting of each port. When any one of ports has tx or rx
timestampping enabled, bit 6 of PTP_MSG_CONF1 is set and it is indicated
to tag_ksz.c through tagger bytes. This flag adds 4 additional bytes to
the tail tag.  When tx and rx timestamping of all the ports are disabled,
then 4 bytes are not added.

Tested using hwstamp -i <interface>
Signed-off-by: NArun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com> # mostly api
Signed-off-by: NDavid S. Miller <davem@davemloft.net>