net: stmmac: Fix return value check in qcom_ethqos_probe()

In case of error, the function devm_clk_get() returns ERR_PTR() and
never returns NULL. The NULL test in the return value check should be
replaced with IS_ERR().

Fixes: a7c30e62 ("net: stmmac: Add driver for Qualcomm ethqos")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Vinod Koul <vkoul@kernel.org>
Acked-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>