mac80211: get correct default channel width for S1G

mac80211: get correct default channel width for S1G

When deleting a channel context, mac80211 would assing
NL80211_CHAN_WIDTH_20_NOHT as the default channel width.
This is wrong in S1G however, so instead get the allowed
channel width for a given channel.

Fixes eg. configuring strange (20Mhz) width during a scan
on the S1G band.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20200922022818.15855-2-thomas@adapt-ip.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>