USB: atm: Use FIELD_SIZEOF, trivial cleanup.

Signed-off-by: NThiago Farina <tfransosi@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/atm/usbatm.c

@@ -1349,7 +1349,7 @@ static int __init usbatm_usb_init(void)
 {
 	dbg("%s: driver version %s", __func__, DRIVER_VERSION);
 
-	if (sizeof(struct usbatm_control) > sizeof(((struct sk_buff *) 0)->cb)) {
+	if (sizeof(struct usbatm_control) > FIELD_SIZEOF(struct sk_buff, cb)) {
 		printk(KERN_ERR "%s unusable with this kernel!\n", usbatm_driver_name);
 		return -EIO;
 	}