net/enic: fix Rx drop counters

net/enic: fix Rx drop counters

rx_no_bufs is a hardware counter of packets dropped on the
interface due to no host buffers and should be used to update
r_stats->imissed counter instead of rx_nombuf.

Include rx_drop in ierrors. rx_drop is incremented if packets
arrive when the receive queue is disabled.

Add a structure and functions for initializing and clearing
software counters. Add count of Rx mbuf allocation failures
(rx_nombuf) as the first counter.

Fixes: fefed3d1e62c ("enic: new driver")

Signed-off-by: John Daley <johndale@cisco.com>