hwmon: (pmbus_core) Fix maximum number of POUT alarm attributes

There are up to three POUT alarm attributes, not two, since cap_alarm was added.

Reported-by: Michele Petracca <mi.petracca@gmail.com>
Cc: stable@vger.kernel.org # 3.0+ [3.0 will need backport]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>