[media] si2157.c: fix frequency range

According with:
	https://www.silabs.com/Support%20Documents/TechnicalDocs/Si2157-short.pdf

The RF input frequency range of this demod is from 42MHz to 870 MHz.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
1 file changed