rfkill: add the WiMAX radio type

Teach rfkill about wimax radios.

Had to define a KEY_WIMAX as a 'key for disabling only wimax radios',
as other radio technologies have. This makes sense as hardware has
specific keys for disabling specific radios.

The RFKILL enabling part is, otherwise, a copy and paste of any other
radio technology.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>