media: uvcvideo: Refactor Power Line Frequency limit selection

media: uvcvideo: Refactor Power Line Frequency limit selection

Move the PLF mapping logic to its own mapping filter function. This
commit does not introduce any new functionality to the logic, it is just
a preparation patch.

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>