media: uvcvideo: Reorganize format descriptor parsing

Format descriptor parsing has grown over time and now mixes parsing of
frame intervals with various quirk handling. Reorganize it to make the
code easier to follow, by parsing frame intervals first, and then
applying fixes and quirks. No functional change is intended.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>