remoteproc: imx_rproc: fix return value check in imx_rproc_addr_init()

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.

Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>