ath: unshare struct ath_bus_ops between ath5k and ath9k

This struct is not used in any common code, and moving it out of
the ath header makes it easier to add more driver specific ops.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>