hwmon: (ad7418) fix checkpatch issues

fixed:
WARNING: simple_strtol is obsolete, use kstrtol instead
+	long temp = simple_strtol(buf, NULL, 10);

ERROR: do not use assignment in if condition
+	if (!(data = kzalloc(sizeof(struct ad7418_data), GFP_KERNEL))) {

ERROR: do not use assignment in if condition
+	if ((err = sysfs_create_group(&client->dev.kobj, &data->attrs)))

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>