Scan 6 GHz channels after change to 6 GHz-allowed regdom

Scan 6 GHz channels after change to 6 GHz-allowed regdom

Drivers will often report regdom changes in the middle of a scan if they
detect during that scan that the regulatory domain has changed. If this
happens and we enter a regdom that supports 6 GHz channels when the
previous one didn't (this often happens in 6 GHz-capable regdoms for
devices after suspend/resume), immediately trigger a 6 GHz-only scan if
we were not able to connect to an AP on a legacy band.

This should significantly improve connection time to 6 GHz AP after
regdom has been reset.

Signed-off-by: Matthew Wang <matthewmwang@chromium.org>