[media] gspca_sunplus: Fix streaming on logitech quicksmart 420

Don't issue a stream stop to the camera at the end of sd_start, this fixes
streaming with this particular model.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>