ASoC: sprd: Fix return value check in sprd_mcdt_probe()

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

Fixes: d7bff893 ("ASoC: sprd: Add Spreadtrum multi-channel data transfer support")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>