net: dsa/bcm_sf2: move pause mode setting into mac_link_up()

bcm_sf2 only appears to support pause modes on RGMII interfaces (the
enable bits are in the RGMII control register.)  Setup the pause modes
for RGMII connections.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>