gianfar: Simplify skb resources freeing code

Remove dma_free_coherent() from stop_gfar() and gfar_start() calls,
place it into free_skb_resources(). That makes SKB resources management
more understandable, plus free_skb_resources() will be used as a cleanup
routine for gfar_alloc_skb_resources() that will be implemented soon.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>