macvtap: avoid hash calculating for single queue

We decide the rxq through calculating its hash which is not necessary
if we only have one rx queue. So this patch skip this and just return
queue 0. Test shows 22% improving on guest rx pps.

Before: 1201504 pkts/s
After:  1472731 pkts/s

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>