staging: octeon-ethernet: consolidate ndo_open functions

ndo_open for rgmii, sgmii and xaui are almost identical. Put the common
code in a single function.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>