net: macb: Remove dead code

macb_64b_desc is always called when HW_DMA_CAP_64B is defined.
So the return NULL can never be reached. Remove the dead code.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>