gianfar: Fix alloc_skb_resources on -ENOMEM cleanup path

Should gfar_init_bds() return with -ENOMEM inside gfar_alloc_skb_resources(),
free_skb_resources() will be called twice in a row on the "cleanup" path,
leading to duplicate kfree() calls for rx_|tx_queue->rx_|tx_skbuff resulting
in segmentation fault.
This patch prevents the segmentation fault to happen in the future
(rx_|tx_sbkbuff set to NULL), and corrects the error path handling
for gfar_init_bds().

Cc: Paul Gortmaker <paul.gortmaker@windriver.com>

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Acked-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>