hwmon: (pmbus/adm1275) Accept negative page register values

A negative page register value means that no page needs to be
selected. This is used by status register read operations and needs
to be accepted. The failure to do so so results in missed status
and limit registers.

Fixes: da8e48ab

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