MFC: r251715

All of Oxford/PLX OX16PCI954, OXm16PCI954 and OXu16PCI954 share the
exact same (subsystem) device and vendor IDs. However, the reference
design for the OXu16PCI954 uses a 14.7456 MHz clock (as does the EXSYS
EX-41098-2 equipped with these), while at least the OX16PCI954 defaults
to a 1.8432 MHz one. According to the datasheets of these chips, the
only difference in PCI configuration space is that OXu16PCI954 have
a revision ID of 1 while the other two are at 0. So employ the latter
for determining the default clock rates of this family.