net/enic: fix packet loss after MTU change

net/enic: fix packet loss after MTU change

If multiple Rx queues and Rx Scatter are used and the MTU is
modified so that the number of mbufs per packet changes, packet
loss is possible.

The enic completion queue index was miscalculated leaving the
upper half of the queues uninitialized after an MTU change, possibly
leading to completions on those queues not getting processed.

Fixes: c3e09182bcd6 ("net/enic: support scatter Rx in MTU update")
Cc: stable@dpdk.org
Signed-off-by: John Daley <johndale@cisco.com>