mlxsw: spectrum_router: Drop unnecessary WARN_ON_ONCE()

In case the register access failed an error would be logged anyway, so
we can drop the warning.

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