ath9k: Fix key cache slot selection for splitmic

ath_reserve_key_cache_slot() was obviously supposed to return an index
to a free slot, not reserved one. This could have caused problems with
hardware revisions that use splitmic.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>