ath9k: remove the two wiphys scanning at the same time message

When issuing two consecutive scans you could often end up
getting in the logs:

"ath9k: Two wiphys trying to scan at the same time"

This message is due to a race in mac80211 but addressing
that race requires some more major changes on the driver
and perhaps optimizations on mac80211 like removing the
scan complete callback alltogether. Its too late to address
this this kernel release so supress the complaint and annotate
this needs fixing for later.

Cc: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>