[IWLWIFI]: Not correctly dealing with hotunplug.

It makes no sense to enable interrupts if a device has been unplugged.
In addition if in doubt IRQ_HANDLED should be returned.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Acked-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>