net: dsa: bcm_sf2: put device node before return

Put the device node dn before return error code on failure path.

Fixes: 461cd1b0

 ("net: dsa: bcm_sf2: Register our slave MDIO bus")
Signed-off-by: Pan Bian <bianpan2016@163.com>
Link: https://lore.kernel.org/r/20210121123343.26330-1-bianpan2016@163.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>