cfg80211: fix scan crash on single-band cards

commit 58389c69150e6032504dfcd3edca6b1975c8b5bc
Author: Johannes Berg <johannes.berg@intel.com>
Date:   Mon Jul 18 18:08:35 2011 +0200

     cfg80211: allow userspace to control supported rates in scan

made single-band cards crash since it would always
access all wiphy->bands[]. Fix this and reject any
attempts in the new helper ieee80211_get_ratemask()
to do the same, rejecting rates configuration for
unsupported bands.

Reported-by: Pavel Roskin <proski@gnu.org>
Tested-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>