net: macb: fix NULL dereference due to no pcs_config method

This patch fixes NULL pointer dereference due to NULL pcs_config
in pcs_ops.

Fixes: e4e143e2

 ("net: macb: add support for high speed interface")
Reported-by: Nicolas Ferre <Nicolas.Ferre@microchip.com>
Link: https://lore.kernel.org/netdev/2db854c7-9ffb-328a-f346-f68982723d29@microchip.com/
Signed-off-by: Parshuram Thombare <pthombar@cadence.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/1604599113-2488-1-git-send-email-pthombar@cadence.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>