PCI: dwc: Return directly when num-lanes is not found

The num-lanes is optional since it is not needed on some platforms
that bring up the link in firmware.

The link programming is based on the num-lanes properties (which is
optional); if it is not present code must return instead of fiddling
with the lanes value to print an error message.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>