[media] dvb-frontends/cxd2841er: TS_SERIAL config flag

Some constellations work/need a serial TS transport mode. This adds a flag
that will toggle set up of such mode.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Acked-by: Abylay Ospan <aospan@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>