v4l2: Avoid code duplication

The function gst_v4l2_object_add_interlace_mode() has repeating code so
it's best use a loop instead. That will make it easy and simple to add
additional interlace modes in a following patch.
10 jobs for v4l2-cleanup in 39 minutes and 51 seconds (queued for 4 minutes and 32 seconds)
Status Job ID Name Coverage
  Preparation
passed #65613
gst indent

00:00:24

passed #65612
manifest

00:00:24

 
  Build
passed #65614
build fedora x86_64

00:06:06

 
  Test
passed #65615
check fedora

00:15:02

 
  Full Builds
passed #65616
build android arm64 api28

00:04:37

passed #65831
build cerbero fedora x86_64

00:13:16

failed #65732
build cerbero fedora x86_64

00:04:34

failed #65702
build cerbero fedora x86_64

00:01:10

failed #65617
build cerbero fedora x86_64

00:01:46

failed #65715
build cerbero fedora x86_64

00:00:23