cfg80211: only allow S1G channels on S1G band

cfg80211: only allow S1G channels on S1G band

As discovered by syzbot, cfg80211 was accepting S1G
channel widths on non-S1G bands. Add a check for this, and
consolidate the 1MHz frequency check as it ends up being a
subset of the others.

Reported-by: syzbot+92715a0eccd6c881bc32@syzkaller.appspotmail.com
Fixes: 11b34737b18a ("nl80211: support setting S1G channels")
Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20201005165122.17583-1-thomas@adapt-ip.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>