net: phylink: avoid oops during initialisation

If we intend to use PCS operations, mac_pcs_get_state() will not be
implemented, so will be NULL. If we also intend to register the PCS
operations in mac_prepare() or mac_config(), then this leads to an
attempt to call NULL function pointer during phylink_start(). Avoid
this, but we must report the link is down.

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