classsearch for a PCI device with this class designation
from Previous PCI device found in search, or NULL for new search.
Iterates through the list of known PCI devices. If a PCI device is
found with a matching class, a pointer to its device structure is
returned. Otherwise, NULL is returned.
A new search is initiated by passing NULL to the from argument.
Otherwise if from is not NULL, searches continue from next device
on the global list.