[media] gspca: fix setting frame interval type in vidioc_enum_frameintervals()

Set the frame _interval_ type to V4L2_FRMIVAL_TYPE_DISCRETE instead of
using V4L2_FRMSIZE_TYPE_DISCRETE which is meant for frame _size_.

The old and new values happen to be the same so there is no functional
change.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>