rfkill: add the WWAN radio type

Unfortunately, instead of adding a generic Wireless WAN type, a technology-
specific type (WiMAX) was added.  That's useless for other WWAN devices,
such as EDGE, UMTS, X-RTT and other such radios.

Add a WWAN rfkill type for generic wireless WAN devices.  No keys are added
as most devices really want to use KEY_WLAN for WWAN control (in a cycle of
none, WLAN, WWAN, WLAN+WWAN) and need no specific keycode added.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Cc: Iñaky Pérez-González <inaky.perez-gonzalez@intel.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>