[media] ov9640: avoid calling ov9640_res_roundup() twice

Simplify ov9640_s_fmt and ov9640_set_fmt

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>