hwmon: (adm1031) Fix register overwrite in set_fan_div()

Don't rely on the register cache when setting a new fan clock divider.
For one thing, the cache might not have been initialized at all if the
driver has just been loaded. For another, the cached values may be old
and you never know what can happen in the driver's back.

Also invalidate the cache instead of trying to adjust the measured fan
speed: the whole point of changing the clock divider is to get a better
reading.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>