net: dsa: bcm_sf2: fix BCM4908 RGMII reg(s)

BCM4908 has only 1 RGMII reg for controlling port 7.

Fixes: 73b7a604 ("net: dsa: bcm_sf2: support BCM4908's integrated switch")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>