mlxsw: spectrum: Add missing error code on allocation failure

We accidentally return success if the kmalloc_array() call fails.

Fixes: 0e14c777

 ("mlxsw: spectrum: Add the multicast routing hardware logic")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Yotam Gigi <yotamg@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>