rfkill: add __must_check annotations

rfkill is not a small, mere detail in wireless support.  Once it starts
supporting rfkill and users start counting on that support, a wireless
device is at risk of operating in dangerous conditions should rfkill
support fail to properly activate.

Therefore, add the required __must_check annotations on some key functions
of the rfkill API, for which the wireless drivers absolutely MUST handle
the failure mode safely in order to avoid a potentially dangerous situation
where the wireless transmitter is left enabled when the user don't want it
to.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Cc: Matthew Garrett <mjg@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>