Fix PNP matching for iflib NIC drivers

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