usb: dwc3: Fix non-usb3 configurations

Missing nodes may also be signaled via -ENODATA. We need to check for
that to prevent failing in non-usb3 setups.

Furthermore, dev.phy must be NULL'ed in case usb3-phy was not found.

Fixes: 142d50fb ("usb: dwc3: Add support for usb3-phy PHY configuration")
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>