ALSA: n64: Fix return value check in n64audio_probe()

ALSA: n64: Fix return value check in n64audio_probe()

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

Fixes: 1448f8acf4cc ("sound: Add n64 driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Lauri Kasanen <cand@gmx.com>
Link: https://lore.kernel.org/r/20210224013803.2146953-1-weiyongjun1@huawei.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>