ath9k: Fix set_key error codes

Return -EOPNOTSUPP if the algorithm is not supported and -ENOSPC if
there is no room in the key cache. This avoids KERN_ERR printk in
mac80211 for "errors" that are actually expected to happen in normal
operating conditions.

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