staging: et131x: Add link status to adapter struct

This will be used to determine if the link state has changed when the phydev
informs the et131x that a change has occurred (in et131x_adjust_link)

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>