media: imx-mipi-csis: Simplify access to source pad

media: imx-mipi-csis: Simplify access to source pad

The mipi_csis_calculate_params() function needs to access the pad of the
connected source. The pad is already available in csis->source.pad, but
the function takes a convoluted path by getting the pad index and
indexing the source subdev's pads array. Simplify it.

No functional change is intended.

Link: https://lore.kernel.org/r/20250822002734.23516-3-laurent.pinchart@ideasonboard.com
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>