The fec_halt() never free'd both RX and TX DMA descriptors that
were allocated in fec_init(), nor did it free the RX buffers.
Rework the FEC driver so that these descriptors and buffers are
allocated only once in fec_probe().

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>