net: fec: make rx skb handling more robust

Allocate, and then map the receive skb before writing any data to the
ring descriptor or storing the skb.  When freeing the receive ring
entries, unmap and free the skb, and then clear the stored skb pointer.

This means we have ring data and skb pointer in one of two states:
either both fully setup, or nothing setup.

This simplifies the cleanup, as we can use just the skb pointer to
indicate whether the descriptor is setup, and thus avoids potentially
calling dma_unmap_single() on a DMA error value.

Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>