liquidio: fix VF driver off-by-one bug when setting ethtool -C ethX rx-frames

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