hwmon: (pmbus/mp2975) Fix driver initialization for MP2975 device

The commit 1feb31e8 ("hwmon: (pmbus/mp2975) Simplify VOUT code")
has introduced a bug that makes it impossible to initialize MP2975
device:
"""
mp2975 5-0020: Failed to identify chip capabilities
i2c i2c-5: new_device: Instantiated device mp2975 at 0x20
i2c i2c-5: delete_device: Deleting device mp2975 at 0x20
"""
Since the 'read_byte_data' function was removed from the
'pmbus_driver_info ' structure the driver no longer reports correctly
that VOUT mode is direct. Therefore 'pmbus_identify_common' fails
with error, making it impossible to initialize the device.

Restore 'read_byte_data' function to fix the issue.

Tested:
- before: it is not possible to initialize MP2975 device with the
'mp2975' driver,
- after: 'mp2975' correctly initializes MP2975 device and all sensor
data is correct.

Fixes: 1feb31e8 ("hwmon: (pmbus/mp2975) Simplify VOUT code")
Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>
Link: https://lore.kernel.org/r/20240126205714.2363-1-aladyshev22@gmail.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/pmbus/mp2975.c

@@ -126,6 +126,22 @@ static const struct regulator_desc __maybe_unused mp2975_reg_desc[] = {
 
 #define to_mp2975_data(x)  container_of(x, struct mp2975_data, info)
 
+static int mp2975_read_byte_data(struct i2c_client *client, int page, int reg)
+{
+	switch (reg) {
+	case PMBUS_VOUT_MODE:
+		/*
+		 * Enforce VOUT direct format, since device allows to set the
+		 * different formats for the different rails. Conversion from
+		 * VID to direct provided by driver internally, in case it is
+		 * necessary.
+		 */
+		return PB_VOUT_MODE_DIRECT;
+	default:
+		return -ENODATA;
+	}
+}
+
 static int
 mp2975_read_word_helper(struct i2c_client *client, int page, int phase, u8 reg,
 			u16 mask)
@@ -869,6 +885,7 @@ static struct pmbus_driver_info mp2975_info = {
 		PMBUS_HAVE_IIN | PMBUS_HAVE_IOUT | PMBUS_HAVE_STATUS_IOUT |
 		PMBUS_HAVE_TEMP | PMBUS_HAVE_STATUS_TEMP | PMBUS_HAVE_POUT |
 		PMBUS_HAVE_PIN | PMBUS_HAVE_STATUS_INPUT | PMBUS_PHASE_VIRTUAL,
+	.read_byte_data = mp2975_read_byte_data,
 	.read_word_data = mp2975_read_word_data,
 #if IS_ENABLED(CONFIG_SENSORS_MP2975_REGULATOR)
 	.num_regulators = 1,