ASoC: SOF: imx8: Fix return value check in imx8_probe()

ASoC: SOF: imx8: Fix return value check in imx8_probe()

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

Fixes: f8612cb94e5f ("ASoC: SOF: imx: Add i.MX8 HW support")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20190826120003.183279-1-weiyongjun1@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>