V4L/DVB (13399): firedtv: add missing include, rename a constant

Add #include <dvb_demux.h> for dvb_dmx_swfilter_packets().  This was
already indirectly included via firedtv.h, but don't rely on it.

The 4 bytes which were referred to as FIREWIRE_HEADER_SIZE are actually
the source packet header from IEC 61883-4 (MPEG2-TS data transmission
over 1394), not e.g. the IEEE 1394 isochronous packet header.  So choose
a more precise name.

Also, express the payload size as a preprocessor constant too.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>