DTVRecorder: Fix 32 to 64 bit integer arithmetic in FindPSKeyFrames()

This is reported to fix commercial flagging of PVR-150/300/500 recordings
possibly only affecting 32 bit O/Ss due to a bad conversion from a 32 bit
unsigned int to a 64 bit signed int.

Fixes #12313.

Signed-off-by: Paul Harrison <pharrison@mythtv.org>