liquidio: Added ndo_get_vf_stats support

Added the ndo to gather VF statistics through the PF.

Collect VF statistics via mailbox from VF.

Signed-off-by: Intiyaz Basha <intiyaz.basha@cavium.com>
Signed-off-by: Felix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>