rfkill: detect bogus double-registering (v2)

Detect and abort with -EEXIST if rfkill_register is called twice on the
same rfkill struct.  And WARN_ON(it) for good measure.

While at it, flag when we are adding the first switch of a type, we will
need that information later.

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