[media] stv0297: change typecast to u64 to avoid smatch warnings

[media] stv0297: change typecast to u64 to avoid smatch warnings

drivers/media/dvb-frontends/stv0297.c:140 stv0297_get_symbolrate() warn: should 'stv0297_readreg(state, 86) << 8' be a 64 bit type?
drivers/media/dvb-frontends/stv0297.c:141 stv0297_get_symbolrate() warn: should 'stv0297_readreg(state, 87) << 16' be a 64 bit type?
drivers/media/dvb-frontends/stv0297.c:142 stv0297_get_symbolrate() warn: should 'stv0297_readreg(state, 88) << 24' be a 64 bit type?

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