ath: Copy key cache management functions from ath9k to ath

Copied the key cache management functions from ath9k (common.c and hw.c) to
ath/key.c so we can use them from ath5k, later.

Minor changes have been made:
 - renamed ath9k_* to ath_*
 - replaced ah->caps.keycache_size with common->keymax
 - removed ATH9K_IS_MIC_ENABLED since it is always true.
 - the AR_PCU_MIC_NEW_LOC_ENA flag is replaced with (splitmic == 0).

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>