mac80211: don't calculate duration for S1G

For now just skip the duration calculation for frames
transmitted on the S1G band and avoid a warning.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20200922022818.15855-11-thomas@adapt-ip.com


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