rt2x00: Make RFKILL enabled by default

RFKILL should be enabled for _all_ hardware whether
or not they feature a rfkill button or not.

Remove driver specific RFKILL configuration options
and make the rt2x00lib version depend on CONFIG_RFKILL
and defaulting to 'y' to make sure it will always
be enabled when RFKILL was enabled.

This also fixes some bugs where RFKILL wasn't initialized
and didn't respond to RFKILL key presses.

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