ath9k: remove the WARN_ON that triggers if generating a beacon fails

During teardown, mac80211 will not return a new beacon. This is normal and
handled properly in the driver, so there's no need to spam the user with a kernel
warning here.

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