net: bcmgenet: Avoid calling platform_device_put() twice in bcmgenet_mii_exit()

Remove platform_device_put() call after platform_device_unregister()
from function bcmgenet_mii_exit(), otherwise, we will call
platform_device_put() twice.

Fixes: 9a4e7969

 ("net: bcmgenet: utilize generic Broadcom UniMAC MDIO controller driver")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Doug Berger <opendmb@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>