staging: vt6655: remove unnecessary variable skb_dma

skb_dma flips from 0 to the contents buf_dma.

This is nolonger necessary so use buf_dma directly
and remove skb_dma altogether.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>