net: fec: fix missing kmalloc() failure check in fec_enet_alloc_buffers()

fec_enet_alloc_buffers() assumes that kmalloc() will never fail, which
is an invalid assumption.  Fix this by implementing a common error
cleanup path, and use it to also clean up after failed bounce buffer
allocation.

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>