hwmon: (w83795) Fix PWM duty cycle frequency attributes

The PWM duty cycle frequenty attributes are improperly named
(fanN_div instead of pwmN_div) and contain raw values instead of
actual frequencies. Rename them and fix their contents.

Also improve the logic when the user asks for a new frequency, to
always pick the closest supported frequency. The algorithm could
certainly be optimized, but the operation is infrequent enough that
I don't think it's worth the effort.

Signed-off-by: Jean Delvare <khali@linux-fr.org>