[media] dvb-frontends/cxd2841er: add variable for configuration flags

Throughout the patch series some configuration flags will be added to the
demod driver. This patch prepares this by adding the flags var to
struct cxd2841er_config, which will serve as a bitmask to toggle various
options and behaviour in the driver.

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>