ath9k_hw: add a new register op for read-mask-write

Reduces the number of calls to register ops. On MIPS this reduces the
ath9k_hw binary size from 321k down to 310k

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