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: ecadcc47 ("remoteproc: imx_rproc: use devm_ioremap")
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210312080420.277151-1-weiyongjun1@huawei.comSigned-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>