hwmon: (pmbus/max31785) check for partial i2c_transfer in read_long_data

hwmon: (pmbus/max31785) check for partial i2c_transfer in read_long_data

i2c_transfer() returns the number of messages successfully
transferred, not only a negative errno on failure. When called with
two messages (write command byte followed by a read of the 4-byte
response), a return value of 1 means the command write succeeded but
the read did not complete. In that case, rspbuf remains uninitialized
and must not be interpreted as valid data.

Treat any return value other than ARRAY_SIZE(msg) as an error, and
return -EIO for partial completion. Also return 0 on success instead
of the message count, since the caller only needs to distinguish
success from failure.

Signed-off-by: Sanman Pradhan <psanman@juniper.net>
Link: https://lore.kernel.org/r/20260321181052.27129-4-sanman.pradhan@hpe.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>