[media] ivtv: use for_each_sg

Use for_each_sg() instead of open-coding it.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>