smsc9420: add ethtool register dump support

This patch adds support for SMSC's LAN9420 PCI ethernet controller
to ethtool's dump registers (-d) command.

This patch is for use with an accompanying ethtool patch, which decodes
the register dump.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>