Take care to handle register 0xa228 exactly as in the HAL released by
Atheros. This change is required to make ath5k work again on my system
since commit 2203d6be (ath5k: Misc hw_reset updates), thus fixing a
regression in 2.6.27 and therefore hopefully eligible for inclusion into
a stable release.

v2: Only overwrite initial register values on later revisions of AR5212
    chips.
v3: Use standard macros to manipulate the register.
Signed-off-by: Elias Oltmanns <eo@nebensachen.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>