tg3: tg3_timer() should grab tp->lock before checking for tp->irq_sync

This is to avoid the race between tg3_timer() and the execution paths
which does not invoke tg3_timer_stop() and releases tp->lock before
calling synchronize_irq()

Reported-by: Peter Hurley <peter@hurleysoftware.com>
Tested-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Prashant Sreedharan <prashant@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>