ath9k_hw: remove ATH9K_CAP_MCAST_KEYSRCH

The driver always sets this to enabled, but this can be simplified with
a small change to ah->sta_id1_defaults instead.
This change also removes the now-obsolete ath9k_hw_setcapability function.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>