cfg80211: add a helper to identify 6 GHz PSCs

This allows identifying whether or not a channel is a PSC
(preferred scanning channel).

Link: https://lore.kernel.org/r/20200528213443.414363ecf62c.Ic15e681a0e249eab7350a06ceb582cca8bb9a080@changeid


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