Replace common->splitmic with ATH_CRYPT_CAP_MIC_COMBINED flag.

splitmic has to be used when the ATH_CRYPT_CAP_MIC_COMBINED capability flag is
not set.
Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>