ath9k: remove duplicate get_hw_crypto_keytype()

Use ath9k_cmn_get_hw_crypto_keytype() instead which is
already exported and shared, and does exactly the same thing.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>