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>

drivers/hwmon/pmbus/max34440.c

@@ -93,12 +93,14 @@ static int max34440_write_word_data(struct i2c_client *client, int page,
 
 static int max34440_read_byte_data(struct i2c_client *client, int page, int reg)
 {
-	int ret;
+	int ret = 0;
 	int mfg_status;
 
-	ret = pmbus_set_page(client, page);
-	if (ret < 0)
-		return ret;
+	if (page >= 0) {
+		ret = pmbus_set_page(client, page);
+		if (ret < 0)
+			return ret;
+	}
 
 	switch (reg) {
 	case PMBUS_STATUS_IOUT: