rfkill: add type string helper

We will need access to the rfkill switch type in string format for more
than just sysfs.  Therefore, move it to a generic helper.

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>