media: nuvoton: npcm-video: Fix IS_ERR() vs NULL bug

The of_parse_phandle() function returns NULL on error.  It never returns
error pointers.  Update the check accordingly.

Fixes: 70721089985c ("media: nuvoton: Add driver for NPCM video capture and encoding engine")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>