[media] gscpa - sn9c20x: Add sd_isoc_init ensuring enough bw when i420 fmt

When using the SN9C20X_I420 fmt the sn9c20x needs more bandwidth
than our regular bandwidth calculations reserve.

This patch adds a sd_isoc_init function, which forces the use of a specific
altsetting when using the SN9C20X_I420 fmt.

This fixes the bottom 10-30% of the image getting corrupted when using
the SN9C20X_I420 fmt (which is the default fmt).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>