liquidio: fix PF falsely indicating success at setting MAC address of a nonexistent VF

In the function assigned to .ndo_set_vf_mac, check the validity of the
vfidx argument before proceeding to tell the firmware to set the VF MAC
address.

Signed-off-by: Felix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: Derek Chickles <derek.chickles@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>