cfg80211/mac80211: fill qual.qual value/adjust max_qual.qual

Due to various bugs in the software stack we end up having
to fill qual.qual; level should be used, but wpa_supplicant
doesn't properly ignore qual.qual, NM should use qual.level
regardless of that because qual.qual is 0 but doesn't handle
IW_QUAL_DBM right now.

So fill qual.qual with the qual.level value clamped to
-110..-40 dBm or just the regular 'unspecified' signal level.
This requires a mac80211 change to properly announce the
max_qual.qual and avg_qual.qual values.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>