ath5k: unify rx descriptor error handling

There is no reason for a special handling (return) here, just break like we do
with the checks before.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>