avformat/mov: fix timecode with counter mode flag set

avformat/mov: fix timecode with counter mode flag set

The current behaviour ends up squaring the avg_frame_rate if the conter mode flag is set.
This messes up the timecode calculation, and looks to me as a regression that
seems to have been introduced 428b4aac.

Upon further testing is seems that no special case is need for having the counter flag set.
av_timecode_init appears to handles the timecode correctly, at least in the sample files
I have.

Here is a sample mov file with the counter flag set
https://www.dropbox.com/s/5l4fucb9lhq523s/timecode_counter_mode.mov

before the patch ffmpeg will report the timecode as:
00:37:11:97 and warns that the timecode framerate is 576000000/1002001

after patch:
14:50:55:02

Signed-off-by: Anton Khirnov <anton@khirnov.net>