[media] msi2500: revise synthesizer calculation

Update synthesizer calculation to model I prefer nowadays. It is mostly
just renaming some variables, but also minor functionality change how
integer and fractional part are divided (using div_u64_rem()). Also, add
'schematic' of synthesizer following my current understanding.

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