[media] lmedm04: use u32 instead of u64 for relative stats

Cleanup this sparse warning:
	drivers/media/usb/dvb-usb-v2/lmedm04.c:302 lme2510_update_stats() warn: should '((255 - st->signal_sn - 161) * 3) << 8' be a 64 bit type?

Both c_tmp and s_tmp actually stores a u16 stat. Using a u64 data
there is a waste, specially on u32 archs, as 64 ints there are more
expensive.

So, change the types to u32 and do the typecast only when storing
the result.

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