gianfar: fix the last transmit buffer descriptor

When the transmit hardware timestamping is enabled, an additional
TxBD would be added and would be set as the last TxBD with TXBD_LAST
and TXBD_INTERRUPT. However this has been broken by a patch recently.
This made the software couldn't get transmit hardware timestamps and
resulted in call trace. So, this patch is to fix this issue.

Fixes: 48963b44 ("gianfar: Remove redundant ops for do_tstamp
       from xmit()")
Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>