mac80211: fix channel selection bug

When trying to connect to an AP that advertises HT but not
VHT, the mac80211 code erroneously uses the configuration
from the AP as is instead of checking it against regulatory
and local capabilities. This can lead to using an invalid
or even inexistent channel (like 11/HT40+).

Additionally, the return flags from downgrading must be
ORed together, to collect them from all of the downgrades.
Also clarify the message.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>