[media] ov9740: avoid calling ov9740_res_roundup() twice

Simplify ov9740_s_fmt.

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