mlxsw: spectrum_router: Use correct tree index for binding

By a mistake, there is tree index 0 passed to RALTB. Should be
MLXSW_SP_LPM_TREE_MIN.

Fixes: b45f64d1

 ("mlxsw: spectrum_router: Use FIB notifications instead of switchdev calls")
Reported-by: Yotam Gigi <yotamg@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>