media: gspca: support multiple pixel formats in ENUM_FRAMEINTERVALS

If a driver supports multiple pixel formats with the same frame size,
ENUM_FRAMEINTERVALS will currently only work for the first pixel format.
Fix this by adding pixelformat support to wxh_to_mode().

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>