selftests: forwarding: ethtool_mm: support devices with higher rx-min-frag-size

Some devices have errata due to which they cannot report ETH_ZLEN (60)
in the rx-min-frag-size. This was foreseen of course, and lldpad has
logic that when we request it to advertise addFragSize 0, it will round
it up to the lowest value that is _actually_ supported by the hardware.

The problem is that the selftest expects lldpad to report back to us the
same value as we requested.

Make the selftest smarter by figuring out on its own what is a
reasonable value to expect.

Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Roger Quadros <rogerq@kernel.org>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>