net: mvpp2: check ethtool sets the Tx ring size is to a valid min value

This patch fixes the Tx ring size checks when using ethtool, by adding
an extra check in the PPv2 check_ringparam_valid helper. The Tx ring
size cannot be set to a value smaller than the minimum number of
descriptors needed for TSO.

Fixes: 1d17db08

 ("net: mvpp2: limit TSO segments and use stop/wake thresholds")
Suggested-by: Yan Markman <ymarkman@marvell.com>
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>