usb: dwc3: gadget: Handle 0 xfer length for OUT EP

For OUT endpoints, zero-length transfers require MaxPacketSize buffer as
per the DWC_usb3 programming guide 3.30a section 4.2.3.3.

This patch fixes this by explicitly checking zero length
transfer to correctly pad up to MaxPacketSize.

Fixes: c6267a51 ("usb: dwc3: gadget: align transfers to wMaxPacketSize")
Cc: stable@vger.kernel.org

Signed-off-by: Tejas Joglekar <joglekar@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>