hwmon: (adm1029) Ensure the fan_div cache is updated in set_fan_div

commit 1035a9e3 upstream.

Writing to fanX_div does not clear the cache. As a result, reading
from fanX_div may return the old value for up to two seconds
after writing a new value.

This patch ensures the fan_div cache is updated in set_fan_div().

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>