MBO: Always include Non-preferred Channel Report attribute in AssocReq

MBO: Always include Non-preferred Channel Report attribute in AssocReq

Include the Non-preferred Channel Report attribute in (Re)Association
Request frames even when the MBO STA has no non-preferred channels in
any operating classes. In case of no non-preferred channels the
attribute length field shall be set to zero and the Operating Class,
Channel List, Preference and Reason Code fields shall not be included.
This indicates to the MBO AP that the MBO STA has no non-preferred
channels access all supported operating classes.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>