[media] msi3101: few improvements for RF tuner

* Fix AM_MODE. Now it could work at least in theory, cannot test.
* Use greatest common divisor algo to divide PLL fractional parts.
* Fix IF frequency mode.
* + some very minor "style" issues

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