sfc: Store the efx_nic struct of the current VF in the VF data struct

Initialised in efx_probe_vf and removal is dealt with in
efx_ef10_remove.

vf->efx is needed in future patches to change the MAC address
of the VF via the parent PF, while the driver is bound to the
VF.
Example: ip link set dev vf NUM mac LLADDR

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