fec: fix uninitialized rx buffer usage

The fec driver was enabling receive buffer descriptor without allocating
the buffers. Make sure the buffer descriptors are initialized to not
start receiving packets.

Open also calls fec_restart after the rx buffers are allocated. With the code
in fec_restart, it zeroes out the buffer descriptors that have just been
setup.

Signed-off-by: Rob Herring <r.herring@freescale.com>
Signed-off-by: Amit Kucheria <amit.kucheria@canonical.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>