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: NPan Bian <bianpan2016@163.com>
Link: https://lore.kernel.org/r/20210121123343.26330-1-bianpan2016@163.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>