net: fec: ensure fec_enet_free_buffers() properly cleans the rings

Ensure that we do not double-free any allocations, and that any transmit
skbuffs are properly freed when we clean up the rings.

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>