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>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>