media: staging/imx7: Fix an error code in mipi_csis_clk_get()

We accidentally return IS_ERR(), which is 1, instead of the PTR_ERR()
which is the negative error code.

Fixes: 7807063b ("media: staging/imx7: add MIPI CSI-2 receiver subdev for i.MX7")

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>