hwmon: (ds1621) Update zbits after conversion rate change

After the conversion rate is changed, the zbits are not updated,
but should be, since they are used later in the set_temp function.

Fixes: a50d9a4d ("hwmon: (ds1621) Fix temperature rounding operations")
Reported-by: Murat Ilsever <murat.ilsever@gmail.com>
Signed-off-by: Robert Coulson <rob.coulson@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>