net: bcmgenet: Fix a resource leak in an error handling path in the probe functin

If the 'register_netdev()' call fails, we must undo a previous
'bcmgenet_mii_init()' call.

Fixes: 1c1008c7

 ("net: bcmgenet: add main driver file")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20201212182005.120437-1-christophe.jaillet@wanadoo.fr


Signed-off-by: Jakub Kicinski <kuba@kernel.org>