rfkill: rename rfkill_mutex to rfkill_global_mutex

rfkill_mutex and rfkill->mutex are too easy to confuse with each other.

Rename rfkill_mutex to rfkill_global_mutex, so that they are easier to tell
apart with just one glance.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Cc: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>