qla3xxx: bugfix tx reset after stress conditions.

To Reproduce the Problem:
To reproduce this panic consistently, we run an intensive network
application like 'netperf' and then switch to a different console.
After waiting for a couple of seconds, you will see a tx reset has occured.

Reason:
We enable interrupts even if we were not running.

Solution:
Now we will enable interrupts only after we are ready to give up the poll
routine.

Signed-off-by: Ron Mercer <ron.mercer@qlogic.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>