selftests: drivers: hw: Fix ethtool_rmon

When rx-pktsNtoM reports a range that involves very low-valued range, such
as 0-64, the calculated length of the packet will be -4, because FCS is
subtracted from the value. mausezahn then confuses the value for an option
and bails out. As a result, the test dumps many mausezahn error messages.

Instead, cap the value at 0. mausezahn will use an appropriate minimum
packet length.

Cc: Vladimir Oltean <vladimir.oltean@nxp.com>
Cc: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/drivers/net/hw/ethtool_rmon.sh

@@ -44,6 +44,7 @@ bucket_test()
 	# Mausezahn does not include FCS bytes in its length - but the
 	# histogram counters do
 	len=$((len - ETH_FCS_LEN))
+	len=$((len > 0 ? len : 0))
 
 	before=$(ethtool --json -S $iface --groups rmon | \
 		jq -r ".[0].rmon[\"${set}-pktsNtoM\"][$bucket].val")