media: staging: media: imx: imx7-mipi-csis: Make subdev name unique

When multiple CSIS instances are present in a single graph, they are
currently all named "imx7-mipi-csis.0", which breaks the entity name
uniqueness requirement. Fix it by using the device name to create the
subdev name.

Fixes: 7807063b ("media: staging/imx7: add MIPI CSI-2 receiver subdev for i.MX7")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Tested-by: Jerome Brunet <jbrunet@baylibre.com> # On i.MX8MP
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>