lavf/mxf: fix parsing of timestamps

Correct bit mask for month/day/hour/min/sec values.

For reference the timestamp format specified in S377M is as follow:
  year (int16), month (uint8), day (uint8), hour (uint8), sec (uint8),
  msec (uint8).
A value of 0 for every fields means timestamp "unknown".

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>