sfc: Bind the sfc driver to any available VF's

Add the device ID of the VF to the PCI device ID table.

Added a boolean flag is_vf in efx_nic_type to differentiate
between a VF and PF at probe time. This flag is useful in later
patches while setting MAC address specially in the
PCI-passthrough case.

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>