ath9k_hw: embed the ath_ops callbacks in the ath_hw struct

With this change, loading the address to a register read/write function
costs only one pointer dereference instead of two. On MIPS this reduces
ath9k_hw binary size from 326k down to 321k.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>