rfkill: Use mutex_lock() at register and add sanity check

Replace mutex_lock_interruptible() by mutex_lock() in rfkill_register(),
as interruptible doesn't make sense there.

Add a sanity check for rfkill->type, as that's used for an unchecked dereference
in an array and might cause hard to debug crashes if the driver sets this
to an invalid value.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>