s390/raw3270: fix nullpointer check

Fix the following checkpatch warning:

CHECK: Comparison to NULL could be written "!rp"
+       if (rp == NULL)
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

drivers/s390/char/raw3270.c

@@ -1232,7 +1232,7 @@ static void raw3270_remove(struct ccw_device *cdev)
 	 * devices even if they haven't been varied online.
 	 * Thus, rp may validly be NULL here.
 	 */
-	if (rp == NULL)
+	if (!rp)
 		return;
 
 	sysfs_remove_group(&cdev->dev.kobj, &raw3270_attr_group);