[PATCH] iseries_veth: Set dev->trans_start so watchdog timer works right

Hi Andrew, Jeff,

The iseries_veth driver doesn't set dev->trans_start in it's TX path. This
will cause the net device watchdog timer to fire earlier than we want it to,
which causes the driver to needlessly reset its connections to other LPARs.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>