ath9k_hw: start building an abstraction layer for hardware routines

ath9k supports the AR5008, AR9001 and AR9002 family of Atheros
chipsets, all 802.11n. The new breed of 802.11n chips, the
AR9003 family will be supported as well soon. To help with its
support we're going to add a few callbacks for hardware routines
which differ considerably instead of adding branch checks for
the revision at runtime.

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