net: bcmgenet: add missed clk_disable_unprepare in bcmgenet_probe

The driver forgets to call clk_disable_unprepare() in error path after
a success calling for clk_prepare_enable().

Fix to goto err_clk_disable if clk_prepare_enable() is successful.

Fixes: c80d36ff

 ("net: bcmgenet: Use devm_clk_get_optional() to get the clocks")
Signed-off-by: Zhang Changzhong <zhangchangzhong@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>