iwl4965: fix not correctly dealing with hotunplug

The interrupt handler returns IRQ_NONE if it detects that the device
is gone. That's incorrect because the device may have raised the interrupt.
Not acknowledging it may trigger the spurious interrupt detection and kill
drivers sharing the interrupt line.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>