media: imx: imx7_mipi_csis: Don't set reserved CLK_CTRL field on i.MX8MM

On CSIS v3.6.3, found in the i.MX8MM, the MIPI_CSIS_CLK_CTRL_WCLK_SRC
field is reserved. Don't set it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>