ath9k: move bt_stomp_type to driver core

The bt_stomp_type defines the bt coex weight, it has a one-to-one
mapping. In the future we may want to just use the weight directly.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>