hwmon: (w83627ehf) Store rpm instead of raw fan speed data

Since the fan speed value can be above 0xff, we can no longer use
that value to determine if the fan speed reading is valid. This
makes it difficult to manipulate the stored fan speed register value.

If we store rpm instead of the fan speed register value, we do not
need to correct it if the fan divisor value is changed, and the above
mentioned problem no longer exists.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Ian Dobson <i.dobson@planet-ian.com>