ath9k: fix key allocation error handling for powersave keys

commit 4ef69d0394cba8caa9f75d3f2e53429bfb8b3045 upstream.

If no keycache slots are available, ath_key_config can return -ENOSPC.
If the key index is not checked for errors, it can lead to logspam that
looks like this: "ath: wiphy0: keyreset: keycache entry 228 out of range"
This can cause follow-up errors if the invalid keycache index gets
used for tx.

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