[media] dvb-core: fix 32-bit overflow during bandwidth calculation

Frontend bandwidth calculation overflows on very high DVB-S/S2
symbol rates. Use mult_frac() macro in order to keep calculation
correct.

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