V4L/DVB (10098): ov772x: fix try_fmt calculation method

Don't modify driver's state in try_fmt, just verify format acceptability
or adjust it to driver's capabilities.

Signed-off-by: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>