media: sun6i-csi: Allow the video device to be open multiple times

Previously it was possible, but a recent fix for uninitialized
`ret` variable broke this behavior.

v4l2_fh_is_singular_file() check is there just to determine
whether the power needs to be enabled, and it's not a failure
if it returns false.

Fixes: ba913911 ("media: sun6i-csi: add a missing return code")
Signed-off-by: Ondrej Jirman <megous@megous.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>