hwmon: (max31790) Fix dereference of ERR_PTR

max31790_update_device() return the error code in ERR_PTR. We were
checking if it has returned error or not but before checking we have
dereferenced it.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>