rt2x00: Replace RFKILL with INPUT

As discussed on linux-wireless rt2x00 does not offer a true RFKILL key,
for that reason RFKILL support should be entirely removed.

The key which is attached to the hardware should be treated as normal
input device instead. Implement input_poll_dev support to poll the device
frequently. When the key status has changed report it as a SW event.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>