platform/x86: mlx-platform: free first dev on error

There is an off-by-one error so we don't unregister priv->pdev_mux[0].
Also it's slightly simpler as a while loop instead of a for loop.

Fixes: 58cbbee2 ("x86/platform/mellanox: Introduce support for Mellanox systems platform")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>