net: dsa: bcm_sf2: Utilize nested MDIO read/write

We are implementing a MDIO bus which is behind another one, so use the
nested version of the accessors to get lockdep annotations correct.

Fixes: 461cd1b0 ("net: dsa: bcm_sf2: Register our slave MDIO bus")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>