[media] mn88473: 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

Lets add some reasonable limits in order to keep w_scan happy :)

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