net: rfkill: add rfkill_find_type function

Helper for finding the type based on name. Useful if the
type needs to be determined based on device property.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
[modify rfkill_types array and BUILD_BUG_ON to not cause errors]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>