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/adm1275.c b/drivers/hwmon/pmbus/adm1275.c
index fa18112..980a4d9 100644
--- a/drivers/hwmon/pmbus/adm1275.c
+++ b/drivers/hwmon/pmbus/adm1275.c
@@ -144,7 +144,7 @@
 	const struct adm1275_data *data = to_adm1275_data(info);
 	int mfr_status, ret;
 
-	if (page)
+	if (page > 0)
 		return -ENXIO;
 
 	switch (reg) {