iwlagn: downgrade BUG_ON in interrupt

This BUG_ON really shouldn't trigger, but if it does, as on my machine,
it leaves you wondering what happened because you won't see it. Let's
instead leak a bit of state and memory and at least make it possible to
report it to the kerneloops project to track it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>