ath9k: simplify ath_btcoex_bt_stomp()

The second argument is always the hardware bt coex struct, so
remove it, and rename the function on the path with a ath9k_ prefix.

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