media: i2c: ov5645: Drop fetching the clk reference by name

The OV5645 sensor has a single clock source, so just drop fetching the
clk reference by name.

This is in preparation to drop the "clock-names" property from the DT
binding.

Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>