ath9k_hw: remove pCap->keycache_size

Similar to the number of tx queue, the number of keycache entries depends
on the chip and shouldn't be messed with based on EEPROM data.
Remove this field and stick to using AR_KEYTABLE_SIZE

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