ath5k: fix error handling in ath5k_beacon_send

This cleans up error handling for the beacon in case of dma mapping
failure.  We need to free the skb when dma mapping fails instead of
nulling and leaking the pointer, and we should bail out to avoid
giving the hardware the bad descriptor.

Finally, we need to perform the null check after trying to update
the beacon, or else beacons will never be sent after a single
mapping failure.

Cc: stable@kernel.org
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>