ath5k: clarify srev comparison for CCMP check

As Pavel Roskin noted, the check for mac version as copied from
legacy_hal made no sense.  This replaces it with the equivalent
and makes up a suitable #define for the mac version legacy_hal
checked.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>