ath9k_hw: remove ATH9K_CAP_DS

This capability check is no longer used, so it can be removed along with
the now-obsolete ath9k_hw_getcapability function.

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