ath9k: Clean up setkey operations

There is no need to use ath_keyset() wrapper for
ath9k_hw_set_keycache_entry() calls. In addition, improve the comments
describing the key setting operations.

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