iwlwifi: add LED mode to support different LED behavior

Today's implementation allow LED to blink based on the traffic
condition. We introduce an additional LED mode that reflects the RF
state.

The supported LED modes after this are:
  IWL_LED_BLINK (current/default) - blink rate based on current Tx/Rx
                                    traffic
  IWL_LED_RF_STATE (new) -
    LED OFF: No power/RF disabled, the LED is emitting no light
    LED ON:  Powered/RF enabled, the LED is emitting light
             in a stable non-flashing state.

In order to provide the flexibility to support different LED
behavior per user/system preference we add "led_mode" iwlcore module
parameter.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>