gianfar: Fix packet drop when out of memory

The patch which fixed gianfar so it drops packets when it runs out
of memory left in the code which frees the skb when it drops packets.
Change the code so that we only free the skb if the new skb was successfully
created.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>