ath9k: Add sanity check for beacon_int in adhoc/mesh case

It looks like mac80211 can request the driver to start beaconing with
a beacon interval of zero in some cases (at least for mesh point). This
does not sound correct and something may need to be fixed in
mac80211. However, taken into account the unpleasantness of getting
stuck in an infinite busy loop with rtnl_lock held, let's add a quick
workaround in the driver to avoid the worst symptom while someone more
familiar with the mesh implementation can figure out what should be done
with mac80211 as far as beacon interval configuration is concerned.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>