hwmon: (pmbus/ibm-cffps) Prevent writing on_off_config with bad data

If the user write parameters resulted in no bytes being written to the
temporary buffer, then ON_OFF_CONFIG will be written with uninitialized
data. Prevent this by bailing out in this case.
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/1578411640-16929-1-git-send-email-eajames@linux.ibm.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/pmbus/ibm-cffps.c

@@ -250,7 +250,7 @@ static ssize_t ibm_cffps_debugfs_write(struct file *file,
 		pmbus_set_page(psu->client, 0);
 
 		rc = simple_write_to_buffer(&data, 1, ppos, buf, count);
-		if (rc < 0)
+		if (rc <= 0)
 			return rc;
 
 		rc = i2c_smbus_write_byte_data(psu->client,