rt2x00: use wiphy rfkill interface

Remove the input_polldev from rt2x00 and replace it with
the rfkill interface offered by the wiphy structure. This
simplifies the entire rfkill handling in rt2x00 and allows
us to remove the CONFIG_RT2X00_LIB_RFKILL option and always
enables rfkill capabilities.

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