serial-lcr: move probe, dev inst creation, data read out of scan

serial-lcr: move probe, dev inst creation, data read out of scan

Move the initial device probe (LCR packet validity check), the creation
of the device instance after successful probe, and the subsequent packet
inspection after resource allocation out of the scan routine. This shall
improve readability of the serial-lcr driver's probe logic, and reduces
diffs when handling of multiple connections gets added later.

Add a developer comment, the serial-lcr driver needs to handle multiple
connections when the conn= spec is ambiguous (multiple cables of the
same type, with the same VID:PID).