bnx2: fixing a timout error due not refreshing TX timers correctly

When running the following script on an active bnx2 interface:

while(true); do ifconfig ethX mtu 9000; ifconfig ethX mtu 1500; done

A timeout error appears and dumps the following stack:

NETDEV WATCHDOG: eth4 (bnx2): transmit queue 0 timed out
------------[ cut here ]------------
Badness at net/sched/sch_generic.c:261
<snip>

This patch just fixes the way that ->trans_start is refreshed.

Signed-off-by: Breno Leitao <leitao@linux.vnet.ibm.com>
Acked-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>