netxen: support for ethtool set ringparam

Add support for ethtool -G to tune rx and tx ring sizes
per interface basis.

This is only supported for NX3031 based cards.

Signed-off-by: Amit Kumar Salecha <amit@netxen.com>
Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>