staging: gdm72xx: Set up parent link in sysfs for gdm72xx devices

This patch uses SET_NETDEV_DEV to set up a 'device' parent link in sysfs
(e.g. /sys/class/net/wm0/device) for a gdm72xx device.

Signed-off-by: Paul Stewart <pstew@chromium.org>
Signed-off-by: Ben Chan <benchan@chromium.org>
Cc: Sage Ahn <syahn@gctsemi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>