hwmon: (pmbus) Fix vout margin caching

hwmon: (pmbus) Fix vout margin caching

The code currently uses a zero margin to mean not cached, but this
results in the cache being bypassed if the (low) margin is set to zero,
leading to lots of unnecessary SMBus transactions in that case.  Use a
negative value instead.

Fixes: 07fb76273db89d93 ("hwmon: (pmbus) Introduce and use cached vout margins")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20220816144414.2358974-1-vincent.whitchurch@axis.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>