net: ag71xx: add missed clk_disable_unprepare in error path of probe

The ag71xx_mdio_probe() forgets to call clk_disable_unprepare() when
of_reset_control_get_exclusive() failed. Add the missed call to fix it.

Fixes: d51b6ce4 ("net: ethernet: add ag71xx driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Huang Guobin <huangguobin4@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>