udev-builtin-net_id: split-out pci_get_onboard_index() from dev_pci_onboard()

udev-builtin-net_id: split-out pci_get_onboard_index() from dev_pci_onboard()

This also changes the onboard index type to unsigned, as it is at most
2^16 - 1. Hence, unsigned is enough.

Then, we can use device_get_sysattr_unsigned() helper function.

This also drops mostly redundant debugging logs. The index will be
logged later anyway.