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: NWei Yongjun <weiyongjun1@huawei.com>
Acked-by: NDoug Berger <opendmb@gmail.com>
Acked-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>