media: sun6i-csi: Get bridge subdev directly in capture stream ops

The remote subdev connected to the capture video device is always
our bridge, so get the bridge subdev directly instead of using a
dedicated helper (which is removed by this commit).

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>