ath5k: Fix reset sequence for AR5212 in general and RF5111 in particular

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>