gianfar: Remove redundant ops for do_tstamp from xmit()

Timestamp BD status updates that can be merged into the
same "do_tstamp" block, no need for extra save/restore
to the BD area.  The code is more readable too.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>