usb: dwc3: core: support optional PHYs

Since PHYs for dwc3 is optional (not all SoCs having PHYs for DWC3
should be programmed), do not return from probe if the USB PHY library
returns -ENODEV as that indicates the platform does not have a
programmable PHY.

While this can be considered as a temporary fix, a long term solution
would be to add 'nop' PHY for platforms that does not have programmable
PHY.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>