usb: dwc2: allow dwc2 to get built when USB_GADGET=m

This patch allows the gadget portion of the DWC2 driver to get built when
(!USB && USB_GADGET) condition is encountered.

Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>