Fix PNP matching for iflib NIC drivers

The previous descriptor string specified that all fields were significant for
match. However, the only significant fields for in-tree drivers are
vendor:devid, and the fictitious zero values constructed by PVID() did not
match real subvendor, subdevice, revision, and/or class values, resulting in no
automatic probe.

If a future iflib driver needs to match on other criteria, the descriptor
string can be updated accordingly. (E.g., "V32" and ~0 for unspecified values
in PVID().)

Reported by: mav
Sponsored by: Dell EMC Isilon