hwmon: (lm85) Select the closest PWM frequency

The LM85 and compatible chips only support 8 arbitrary PWM frequencies.
The algorithm to pick one of them based on the user input is not
optimum. Improve it to always pick the closest supported frequency.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Herbert Poetzl <herbert@13thfloor.at>