ieee80211: add code to obtain and parse 6 GHz operation field

Add some code to obtain and parse the 6 GHz operation field
inside the HE operation element.

While at it, fix the required length using sizeof() the new
struct, which is 5 instead of 4 now.

Link: https://lore.kernel.org/r/20200528213443.42ca72c45ca9.Id74bc1b03da9ea6574f9bc70deeb60dfc1634359@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Git-commit: 821273a5a502eebaae005557907d122d1e9b7b98
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


CRs-Fixed: 2743684
Change-Id: Ia0c879d70628189549cb78a4e459d9ee99060b19
Signed-off-by: Vinita S. Maloo <vmaloo@codeaurora.org>