net: lan966x: Fix port configuration when using SGMII interface

In case the interface between the MAC and the PHY is SGMII, then the bit
GIGA_MODE on the MAC side needs to be set regardless of the speed at
which it is running.

Fixes: d28d6d2e

 ("net: lan966x: add port module support")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>