et131x: Add PCIe gigabit ethernet driver et131x to drivers/net

This adds the ethernet driver for Agere et131x devices to
drivers/net/ethernet.

The driver being added has been in the staging tree for some time, and will be
removed from there in a seperate patch. This one merely disables the staging
version to prevent two instances being built.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>