uart/pci: use different probe return values

For PCI devices listed in pci_ns8250_ids return BUS_PROBE_SPECIFIC, while
for generic UART devices not explicitly listed in pci_ns8250_ids return
BUS_PROBE_GENERIC. This allows more specific drivers to take over those
devices, and the generic UART PCI driver will only be used as a fallback.

This fixes an issue where the UART PCI driver would attach to multiport PCI
UART devices, that instead need to use the puc(4) driver to multiplex the
device.

Reported by: markj
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D56467
Reviewed by: markj imp