iwlagn: Re-enable RF_KILL interrupt when down

commit 3dd823e6b86407aed1a025041d8f1df77e43a9c8 upstream.

With commit 554d1d027b19265c4aa3f718b3126d2b86e09a08 only one RF_KILL
interrupt will be seen by the driver when the interface is down.

Re-enable the interrupt when it occurs to see all transitions.

Signed-off-by: Don Fry <donald.h.fry@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>