mlxsw: spectrum_router: Query number of LPM trees from firmware

Instead of hard coding the number of LPM trees in the driver, query it
from the firmware, as it may change in future devices.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>