[media] v4l2: make vidioc_s_modulator const

Write-only ioctls should have a const argument in the ioctl op.
Do this conversion for vidioc_s_modulator.
Adding const for write-only ioctls was decided during the 2012 Media Workshop.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>