net: bcmgenet: Utilize bcmgenet_mii_exit() for error path

bcmgenet_mii_init() has an error path which is strictly identical to the
unwinding that bcmgenet_mii_exit() does, so have bcmgenet_mii_init()
utilize bcmgenet_mii_exit() for that.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>