usb: mtu3: clear u1/u2_enable to 0 in mtu3_gadget_reset

when the device is reset by host, the status of u1_enable and
u2_enable should also be restored to default value.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>