dwc_eth_qos: fix missing clk_disable_unprepare() on error in dwceqos_probe()

Fix missing clk_disable_unprepare() call before return
from dwceqos_probe() in the error handling case of invalid
fixed-link.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>