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>
Git-commit: afbc9c9e8bfe71e8bd12a8c01bedd969fbab8f0e
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


CRs-Fixed: 2743684
Change-Id: Idfaa06479f9812bd1243c06f257b3253e83c8134
Signed-off-by: Vinita S. Maloo <vmaloo@codeaurora.org>