ath9k: fix double-free bug on beacon generate failure

When the next beacon is sent, the ath_buf from the previous run is reused.
If getting a new beacon from mac80211 fails, bf->bf_mpdu is not reset, yet
the skb is freed, leading to a double-free on the next beacon tx attempt,
resulting in a system crash.

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