mac802154: change pan_coord type to bool

To indicate if it's a coordinator or not a bool is enough. There should
no more values available which represent some other state.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reviewed-by: Varka Bhadram <varkabhadram@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>