livetv: dvb subtitle identifiers should be written big endian (fixes 14520)

Thanks @elupus, this works at least for HTSP!

This fixes subtitles when watching live TV (has been broken for half a year).

References:

http://forum.xbmc.org/showthread.php?tid=169386
http://trac.xbmc.org/ticket/14520
https://github.com/xbmc/xbmc/pull/3839#issuecomment-30626519