net: gem: Fix return value from recv

recv function should return 0 instead of frame_len not to
proceed the same packet again in core.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

drivers/net/zynq_gem.c

@@ -569,7 +569,7 @@ static int zynq_gem_recv(struct udevice *dev, int flags, uchar **packetp)
 			priv->rxbd_current = 0;
 	}
 
-	return frame_len;
+	return 0;
 }
 
 static void zynq_gem_halt(struct udevice *dev)