mac80211: BSS info: check channel first

When we receive information about a BSS we check at some point
whether or not we think we're allowed to use the channel it is
on, but we do that fairly late. I don't think we should do it
that late, so do it earlier to avoid doing IBSS/mesh stuff on
that channel and then getting confused because it's disabled.

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