BACKPORT: 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.

Bug: 163110886

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


Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Change-Id: I3044bbe8adee64db9021924f4bf21812566cec07
(cherry picked from commit 821273a5a502eebaae005557907d122d1e9b7b98)
Signed-off-by: Vamsi Krishna <vamsin@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>