iwlagn: Re-enable RF_KILL interrupt when down

With commit 554d1d02 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>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>