usb: dwc3: gadget: Set link state to RX_Detect on disconnect

When DWC3 receives disconnect event, it needs to set the link state to
RX_Detect.

DWC_usb3 3.30a programming guide 4.1.7

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>