can be upgraded to MegaRAID mode, in which case mfi(4) should attach to
these based on the sub-vendor and -device ID instead (not currently done).
Therefore, let mpt_pci_probe() return BUS_PROBE_LOW_PRIORITY.
While it, let mpt_pci_probe() return BUS_PROBE_DEFAULT instead of 0 in
the default case.
MFC after: 3 days