device: Implement a bus agnostic dev_num_vf routine

Now that pci_bus_type has num_vf callback set, dev_num_vf can be
implemented in a bus type independent way and the check for whether a
PCI device is being handled in rtnetlink can be dropped.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>