[media] em28xx: use v4l2_disable_ioctl() to disable ioctls VIDIOC_G_AUDIO and VIDIOC_S_AUDIO

Instead of checking the device type and returning -EINVAL inside the ioctl
functions, use v4l2_disable_ioctl() to disable the ioctls VIDIOC_G_AUDIO and
VIDIOC_S_AUDIO if the device doesn't support audio.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>