ath9k_hw: Use buffered register writes

This patch adds macros at certain places
which could be optimized for multiple register writes.

The performance of ath9k_htc improves considerably,
especially reducing the latency involved in a scan run.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>