ajasrc: Fix handling of timestamps and don't rely on driver frame counters
The driver frame counters (processed, dropped, buffer level) are not always correct apparently, and don't allow reliably assigning a frame number to captured frames.
Instead of relying on them, count the number of frames directly here and detect dropped frames based on the capture times of the frames: if more than 1.75 frame durations are between two frames, then there must've been a dropped frame.