network/ethernet: ioctl returns -1 and stores the error in errno.

Previously, if a device driver returned an error of any kind, -1 was
propagated the rest of the way up through the stack instead of the
actual error code.

Change-Id: I6839763c6b2eb86d6112d3732e6cb80d022f1fe8
Reviewed-on: https://review.haiku-os.org/550
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

src/add-ons/kernel/network/devices/ethernet/ethernet.cpp

@@ -241,7 +241,9 @@ ethernet_control(net_device *_device, int32 op, void *argument,
 	size_t length)
 {
 	ethernet_device *device = (ethernet_device *)_device;
-	return ioctl(device->fd, op, argument, length);
+	if (ioctl(device->fd, op, argument, length) < 0)
+		return errno;
+	return B_OK;
 }