mlxsw: spectrum_router: Mark only first LPM tree as reserved

In new firmware versions (that we can now enforce via
request_firmware()), only the first LPM tree is reserved and not the
first two as in older versions.

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>