ffmpeg: change command line option -dump to work without -loglevel debug

-hex and -dump command line options do nothing unless -loglevel debug is set.

-dump by itself is useful for monitoring live streams (to get the current PTS for example)  however when it is used with -loglevel debug for an RTMP stream, librtmp also dumps the packet data which makes the output too noisy.

do_pkt_dump is only set in check_keyboard_interaction or by the -dump command line option so this change should have no effect on any other parts of the code..

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>