8139cp: Fix DMA unmapping of transmitted buffers

The low 16 bits of the 'opts1' field in the TX descriptor are supposed
to still contain the buffer length when the descriptor is handed back to
us. In practice, at least on my hardware, they don't. So stash the
original value of the opts1 field and get the length to unmap from
there.

There are other ways we could have worked out the length, but I actually
want a stash of the opts1 field anyway so that I can dump it alongside
the contents of the descriptor ring when we suffer a TX timeout.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>