net: dsa: bcm_sf2: Propagate error value from mdio_write

Both bcm_sf2_sw_indir_rw and mdiobus_write_nested could fail, so let's
return their error codes upstream.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>