media: mediatek: vcodec: Improve an error message

This is intended to print the error code but there is a typo so it
prints IS_ERR() instead of PTR_ERR().

Fixes: 77f3b023f452 ("media: mediatek: vcodec: Add debugfs interface to get debug information")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>