net: mvpp2: split the max ring size from the default one

The Rx/Tx ring sizes can be adjusted thanks to ethtool given specific
network needs. This commit splits the default ring size from its max
value to allow ethtool to vary the parameters in both ways.

Signed-off-by: Yan Markman <ymarkman@marvell.com>
[Antoine: commit message]
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>