libertas if_usb: Fix crash on 64-bit machines

On a 64-bit kernel, skb->tail is an offset, not a pointer. The libertas
usb driver passes it to usb_fill_bulk_urb() anyway, causing interesting
crashes. Fix that by using skb->data instead.

This highlights a problem with usb_fill_bulk_urb(). It doesn't notice
when dma_map_single() fails and return the error to its caller as it
should. In fact it _can't_ currently return the error, since it returns
void.

So this problem was showing up only at unmap time, after we'd already
suffered memory corruption by doing DMA to a bogus address.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: stable@kernel.org
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>