rfkill: mutex fixes

There are two mutexes in rfkill:

rfkill->mutex, which protects some of the fields of a rfkill struct, and is
also used for callback serialization.

rfkill_mutex, which protects the global state, the list of registered
rfkill structs and rfkill->claim.

Make sure to use the correct mutex, and to not miss locking rfkill->mutex
even when we already took rfkill_mutex.

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