media: uvcvideo: Increment intervals pointer at end of parsing

The intervals pointer is incremented for each interval when parsing the
format descriptor. This doesn't cause any issue as such, but gets in the
way of constifying some pointers. Modify the parsing code to index the
intervals pointer as an array and increment it in one go at end of
parsing.

Careful readers will notice that the maxIntervalIndex variable is set to
1 instead of n - 2 when bFrameIntervalType has a zero value. This is
functionally equivalent, as n is equal to 3 in that case.

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>