[media] dvb-frontends/cxd2841er: make ASCOT use optional

The Sony CXD28xx demods may have other tuner types attached to them (e.g.
NXP TDA18212), so don't mandatorily configure and enable the ASCOT
functionality, but make this conditional by a config flag.

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>