usb: gadget: mv_udc: enable stream mode

According to ChipIdea's reference manual in section 8.5.2
"Non-streaming operational mode in device mode", we'd better enable stream
mode, especially that ISO endpoints are not supported when the SDIS bit
is set.

Signed-off-by: Neil Zhang <zhangwm@marvell.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>