iwlagn: fix hw-rfkill while the interface is down

Currently iwlagn is not able to report hw-killswitch events while the
interface is down. This has implications on user space tools (like
NetworkManager) relying on rfkill notifications to bring the interface
up once the wireless gets enabled through a hw killswitch.

Thus, enable the device already in iwl_pci_probe instead of iwl_up
and enable interrups while the interface is down in order to get
notified about killswitch state changes. The firmware loading is still
done in iwl_up.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>