usb: dwc2: gadget: replace constants with defines

Defines are more readable and searchable than constants.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>