media: platform: imx-mipi-csis: Remove unneeded 'default n'

There is no need to pass 'default n' in Kconfig, as by default the
CONFIG_VIDEO_IMX_MIPI_CSIS option is not selected.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>