hwmon: (max31790) potential ERR_PTR dereference

We should only dereference "data" after we check if it is an error
pointer.

Fixes: 54187ff9

 ('hwmon: (max31790) Convert to use new hwmon registration API')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>