[media] saa7115: Fix saa711x_set_v4lstd for gm7113c

saa711x_set_v4lstd would toggle several bits that should not be touched
when changing std. This patch fixes this.

Signed-off-by: Jon Arne Jørgensen <jonarne@jonarne.no>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>