atheros/ath9k: add common read/write ops and port ath9k to use it

In an effort to make hw code driver core agnostic read
and write operations are defined on the ath_common structure.
This patch adds that and makes ath9k use it. This allows
drivers like ath9k_htc to define its own read/write ops and
still rely on the same hw code. This also paves the way for
sharing code between ath9k/ath5k/ath9k_htc.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>