hwmon: (pmbus) Fix page vs. register when accessing fans

hwmon: (pmbus) Fix page vs. register when accessing fans

Commit 734e3b8aa171 ("hwmon: (pmbus) Implement multi-phase support")
added support for multi-phase pmbus devices. However, when calling
pmbus_add_sensor() for fans, the patch swapped the `page` and `reg`
attributes. As a result, the fan speeds were reported as 0 RPM on my device.

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Fixes: 734e3b8aa171 ("hwmon: (pmbus) Implement multi-phase support")
Cc: stable@vger.kernel.org # v5.7+
Link: https://lore.kernel.org/r/449bc9e6c0e4305581e45905ce9d043b356a9932.1592904387.git.jan.kundrat@cesnet.cz
[groeck: Fixed references to offending commit]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>