hwmon: (pmbus) Improve boolean handling

Boolean handling depends on storing the sensor data index in sensor_device_attr
as part of the index variable. This limits the number of sensor attributes to
256, and means the sensor sequence number actually has to be maintained to be
able to access sensor data from boolean functions.

Rework the code to store sensor pointers in the pmbus_boolean data structure
directly. With this approach, the number of supportable sensors is now
unlimited.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>