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

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

The commit 1feb31e810b0 ("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: 1feb31e810b0 ("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.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>