net: dsa: bcm_sf2: Get rid of PHYLIB functions

Now that we have converted the bcm_sf2 driver to implement PHYLINK MAC
operations, we can remove the PHYLIB callbacks: adjust_link() and
fixed_link_update() which are no longer called by DSA.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>