Also decrement the reference count of child device on error path.

Fixes: 3e782985 ("net: ethernet: fec: Allow configuration of MDIO bus speed")
Signed-off-by: Pan Bian <bianpan2016@163.com>
Link: https://lore.kernel.org/r/20210120122037.83897-1-bianpan2016@163.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>