media: imx: imx-mipi-csis: Simplify mipi_csis_s_stream()

Simplify the mipi_csis_s_stream() function.

This actually fixes a bug, as if calling the subdev's s_stream(1) fails,
mipi_csis_stop_stream() was not called.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>