[media] si2157: Add support for delivery system SYS_ATSC

Set the property for delivery system also in case of SYS_ATSC. This
behaviour is observed in the sniffs taken with Hauppauge HVR-955Q
Windows driver.

Signed-off-by: Olli Salonen <olli.salonen@iki.fi>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>