usb: dwc3: gadget: Support up to max stream id

DWC3 IPs can use the maximum stream id (up to 2^16) specified by the
USB 3.x specs. Don't limit to stream id 2^15 only. Note that this does
not reflect the number of concurrent streams the controller handles
internally.

Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>