libertas_tf: fix skb tail pointer

skb->tail can't be meant here because it's not the same across 32/64 bit
compilations. This means there's no way the current driver can work on
64-bit architectures.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: stable@kernel.org [2.6.27]
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/libertas_tf/if_usb.c

@@ -331,7 +331,7 @@ static int __if_usb_submit_rx_urb(struct if_usb_card *cardp,
 	/* Fill the receive configuration URB and initialise the Rx call back */
 	usb_fill_bulk_urb(cardp->rx_urb, cardp->udev,
 			  usb_rcvbulkpipe(cardp->udev, cardp->ep_in),
-			  (void *) (skb->tail),
+			  skb_tail_pointer(skb),
 			  MRVDRV_ETH_RX_PACKET_BUFFER_SIZE, callbackfn, cardp);
 
 	cardp->rx_urb->transfer_flags |= URB_ZERO_PACKET;