[media] em28xx: initialize si2168_config struct

When new parameters are added for si2168 driver, the parameters have to be explicitly defined for each device if the
si2168_config struct is not initialized to all zeros.

Signed-off-by: Olli Salonen <olli.salonen@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>