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.

The code is also easier to read with is a bonus.

While here, remove a bogus comment.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>