[media] em28xx-dvb: Prepare for si2157 driver getting more parameters

Modify all users of si2157_config to correctly initialize all not
listed values to 0.

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>