mlxsw: spectrum: Fix check for IS_ERR() instead of NULL

mlxsw_afa_block_create() doesn't return error pointers, it returns NULL
on error.

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>