gianfar: Use gfar_halt to stop DMA in gfar_probe

gfar_halt does everything we want to do there, including disabling
TX/RX.  It also doesn't unnecessarily enable DMA if it's already
stopped.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>