hwmon: (pmbus) Always call _pmbus_read_byte in core driver

Always call _pmbus_read_byte() instead of pmbus_read_byte() in PMBus core
driver. With this change, device specific read functions can be implemented for
all registers.

Since the device specific read_byte function is now always called, we need to be
more careful with page validations. Only fail if the passed page number is larger
than 0, since -1 means "current page".

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>
diff --git a/drivers/hwmon/pmbus/max8688.c b/drivers/hwmon/pmbus/max8688.c
index 7113f11..e2b74bb 100644
--- a/drivers/hwmon/pmbus/max8688.c
+++ b/drivers/hwmon/pmbus/max8688.c
@@ -101,7 +101,7 @@
 	int ret = 0;
 	int mfg_status;
 
-	if (page)
+	if (page > 0)
 		return -ENXIO;
 
 	switch (reg) {