iwlwifi: reintroduce iwl_enable_rfkill_int

If device is disabled by rfkill switch, do not enable all interrupts,
but only CSR_INT_BIT_RF_KILL to receive rfkill state change. Unblocking
other interrupts might cause problems, since driver can not be prepared
for receive them.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>