fec: Fix fec_enet_alloc_buffers() error path

When fec_enet_alloc_buffers() fails we should better undo the previous actions,
which consists of: disabling the FEC clocks and putting the FEC pins into
inactive state.

The error path for fec_enet_mii_probe() is kept unchanged.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>