hwmon: (amc6821) Rename fan1_div to fan1_pulses

The chip does not have a fan divisor. What it does have is a configuration
to set either 2 or 4 pulses per fan rotation. Rename the attribute to
reflect its use. Update documentation accordingly.

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>