[media] lmedm04: fix the range for relative measurements
Relative measurements are typically between 0 and 0xffff. However,
for some tuners (TUNER_S7395 and TUNER_S0194), the range were from
0 to 0xff00, with means that 100% is never archived.
Also, TUNER_RS2000 uses a more complex math.
So, create a macro that does the conversion using bit operations
and use it for all conversions.