usb: dwc2: gadget: Update for new usb_endpoint_maxp()

Update the dwc2 driver for the new behavior of the usb_endpoint_maxp()
and also use the new usb_endpoint_maxp_mult() helper function.

This commit fixes failures in high-badwith ISOC transfer tests.

Signed-off-by: Vardan Mikayelyan <mvardan@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>