net: stmmac: dwxgmac2: remove useless NULL pointer initialisations

Remove useless NULL pointer initialisations for "PCS" methods from the
dwxgmac2 code.

Reviewed-by: Andrew Halaney <ahalaney@redhat.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/E1sCErP-00EOPl-IT@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>