bgmac: don't update slot on skb alloc/dma mapping error

Don't update the slot in "bgmac_dma_rx_skb_for_slot" unless both the
skb alloc and dma mapping are successful; and free the newly allocated
skb if a dma mapping error occurs.  This relieves the caller of the need
to deduce/execute the appropriate cleanup action required when an error
occurs.

Signed-off-by: Nathan Hintz <nlhintz@hotmail.com>
Acked-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>