[media] si2168: define symbol rate limits

w_scan complains about missing symbol rate limits:
This dvb driver is *buggy*: the symbol rate limits are undefined - please report to linuxtv.org

Chip supports 1 to 7.2 MSymbol/s on DVB-C.

Cc: stable@vger.kernel.org
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>