usb: dwc3: ep0: sanity check test mode selector

In case host sends us an unsupported test mode, we
*must* stall this request. This will tell the host
that the selector is invalid and we won't put the
controller in unsupported test modes which could
have undetermined side-effects.

Signed-off-by: Fei Yang <fei.yang@intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>