v4l2: fix enumerating frame interval for continuous/step-wise devices

Created by: gcampax

A device that supports continuous/step-wise frame interval will report the correct frame intervals for index 0 and EINVAL for every other index. (https://linuxtv.org/downloads/v4l-dvb-apis/uapi/v4l/vidioc-enum-frameintervals.html)

Previously, the code would exit from the loop after successfully reading index 0, without marking that the iteration of frame interval completed. Therefore, the next time the function was called the same frame intervals would be read, instead of advancing to the next frame size or format type. Instead, mark that we need to try and read the next format size.

Fixes #56 (closed)