selftests: forwarding: ethtool_rmon: Add histogram counter test

Validate the operation of rx and tx histogram counters, if supported
by the interface, by sending batches of packets targeted for each
bucket.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>