net: phylink: ensure consistent phy interface mode

Ensure that we supply the same phy interface mode to mac_link_down() as
we did for the corresponding mac_link_up() call.  This ensures that MAC
drivers that use the phy interface mode in these methods can depend on
mac_link_down() always corresponding to a mac_link_up() call for the
same interface mode.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>