usb: dwc2: pci: Correctly compile dwc2-pci as a module or built-in

The dwc2-pci driver should be compiled as a module when configured to
do so. If the dwc2-pci is configured as a module but actually
built-in, it can cause build errors due to the fact that the
generic-phy will be allowed to compile as a module causing undefined
references.

Reported-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>