hwmon: (pmbus/max8688) Accept negative page register values

A negative page register value means that no page needs to be
selected. This is used by status register evaluations and needs
to be accepted.

Fixes: da8e48ab

 ("hwmon: (pmbus) Always call _pmbus_read_byte in core driver")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>