usb: dwc2: Don't program DMA address for 0 length request

Check the request length in dwc2_hsotg_start_req() function. If
length == 0, do not write DMA address to control register.

Signed-off-by: Razmik Karapetyan <razmik@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>