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>