iwl3945: streamline iwl3945_rfkill_poll()

iwl3945_rfkill_poll() has been silently calling wiphy_rfkill_set_hw_state()
every 2 seconds, regardless of whether hardware RF KILL switch changed state.

Call wiphy_rfkill_set_hw_state() only when RFKILL switch changes.
Add IWL_DEBUG_RF_KILL log message and documentation.

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>