media: sunxi: sun6i_mipi_csi2.c/sun8i_a83t_mipi_csi2.c: clarify error handling

Both sun6i_mipi_csi2.c and sun8i_a83t_mipi_csi2.c have the same issue:
the comment before the ret = 0 assignment is incorrect, drop it and
always assign the result of the v4l2_subdev_call(..., 0) to ret.

In the disable label check for !on and set ret to 0 in that case.

This fixes two smatch warnings:

drivers/media/platform/sunxi/sun6i-mipi-csi2/sun6i_mipi_csi2.c:193 sun6i_mipi_csi2_s_stream() warn: missing error code 'ret'
drivers/media/platform/sunxi/sun8i-a83t-mipi-csi2/sun8i_a83t_mipi_csi2.c:225 sun8i_a83t_mipi_csi2_s_stream() warn: missing error code 'ret'

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>