media: imx-jpeg: Fix return value check in mxc_jpeg_attach_pm_domains()

In case of error, the function device_link_add() returns NULL
pointer not ERR_PTR(). The IS_ERR() test in the return value
check should be replaced with NULL test.

Fixes: 2db16c6e ("media: imx-jpeg: Add V4L2 driver for i.MX8 JPEG Encoder/Decoder")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>