BACKPORT: cfg80211: add a helper to identify 6 GHz PSCs

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

Bug: 163110886

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


Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Change-Id: Ia0b369ead18ce1d1e585df9467d680be4aa8adef
(cherry picked from commit afbc9c9e8bfe71e8bd12a8c01bedd969fbab8f0e)
Signed-off-by: Vamsi Krishna <vamsin@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>