ASoC: mediatek: mt2701: fix return value check in mt2701_afe_pcm_dev_probe()

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

Fixes: dfa3cbb8 ("ASoC: mediatek: modify MT2701 AFE driver to adapt mfd device")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>