Advertise Supported Operating Classes elements in Beacon/Probe Response

Advertise Supported Operating Classes elements in Beacon/Probe Response

Advertise current operating class in Beacon and Probe Response frames.
This Supported Operating Classes element is required by the standard
when extended channel switch is supported. However, since this element
doesn't reflect correctly the sub-band spectrum breakdown and can't be
effectively used by clients, publish only the minimal required part
which is the current operating class.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>