net: dsa: bcm_sf2: Fix bcm_sf2_reg_rgmii_cntrl() call for non-RGMII port

We cannot call bcm_sf2_reg_rgmii_cntrl() for a port that is not RGMII,
yet we do that in bcm_sf2_sw_mac_link_up() irrespective of the port's
interface. Move that read until we have properly qualified the PHY
interface mode. This avoids triggering a warning on 7278 platforms that
have GMII ports.

Fixes: 55cfeb39 ("net: dsa: bcm_sf2: add function finding RGMII register")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>