hp-wmi: update to match current rfkill semantics

hp-wmi currently changes the RFKill state by altering the struct members
rather than using the dedicated interface, meaning that update events
won't be pushed to userspace.  This patch fixes that, along with fixing
the declared type of the WWAN kill switch.  It also ensures that rfkill
interfaces are only registered for hardware that exists.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Ivo van Doorn <ivdoorn@gmail.com>
Cc: Dave Young <hidave.darkstar@gmail.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>