Instead passing both flags, which can be NULL, and vif_params,
which are never NULL, move the flags into the vif_params and
use BIT(0), which is invalid from userspace, to indicate that
the flags were changed.

While updating all drivers, fix a small bug in wil6210 where
it was setting the flags to 0 instead of leaving them unchanged.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>