ath9k: remove a bogus error message

When beacons are being added or removed for an interface, ieee80211_beacon_get
will sometimes not return a beacon. This is normal and should not result in
useless logspam.

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