remoteproc: qcom: wcss: Fix return value check in q6v5_wcss_init_mmio()

remoteproc: qcom: wcss: Fix return value check in q6v5_wcss_init_mmio()

In case of error, the function devm_ioremap() returns NULL pointer
not ERR_PTR(). The IS_ERR() test in the return value check should
be replaced with NULL test.

Fixes: 0af65b9b915e ("remoteproc: qcom: wcss: Add non pas wcss Q6 support for QCS404")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210319094100.4185044-1-weiyongjun1@huawei.com
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>