ath9k_hw: clean up register write buffering

Throughout the code, DISABLE_REGWRITE_BUFFER is always called right after
REGWRITE_BUFFER_FLUSH. Since that's unlikely to change any time soon, that
makes keeping those ops separate rather pointless, as it only increases
code size and line number counts.

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