[media] si2157: extend frequency range for ATSC

The Si2157 tuner supports ATSC in addition to DVB-T and DVB-C. Extend
minimum frequency range to cover the complete ATSC/QAM-B range.

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