rfkill: document the rfkill struct locking (v2)

Reorder fields in struct rfkill and add comments to make it clear
which fields are protected by 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>