media: video-mux: fix compliance failures

Limit frame sizes to the [1, 65536] interval, media bus formats to
the available list of formats, and initialize pad and try formats.

Reported-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Rui Miguel Silva <rui.silva@linaro.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>