Add checksum offload support for DM9000A and DM9000B chips.

v2 changes: added a local copy of ip_summed to save IO cycles in dm9000_send_packet
v3 changes: trans_start updating is removed.
Signed-off-by: NYeasah Pell <yeasah@comrex.com>
Signed-off-by: NMike Rapoport <mike@compulab.co.il>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>