[media] gspca: fix a v4l2-compliance failure about VIDIOC_ENUM_FRAMEINTERVALS

According to v4l2-compliance VIDIOC_ENUM_FRAMEINTERVALS should fail for
unsupported frame sizes, but gspca is too tolerant and tries to find
the frame intervals for the frame size nearest to the requested one.

This makes v4l2-compliance fail with this message:

  fail: v4l2-test-formats.cpp(123): \
      found frame intervals for invalid size 321x240
  test VIDIOC_ENUM_FMT/FRAMESIZES/FRAMEINTERVALS: FAIL

Fix this by using an exact match for the frame size when enumerating
frame intervals, and retuning an error if the frame size for which the
frame intervals have been asked is not supported.

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>