iwlwifi: fix race condition during driver unload

This patch fixed the OOPS when load the driver while rf-kill is on then
unload the driver right after load. a race condition caused the interupt
handler to schedule the tasklet which will run right after the driver pci_remove
causing invalid poiter OOPS.

Signed-off-by: Mohamed Abbas <mabbas@linux.intel.com>
Signed-off-by: Joonwoo Park <joonwpark81@gmail.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>