mlxsw: spectrum_router: Avoid allocating NH counters twice

mlxsw_sp_nexthop_counter_disable() decays to a nop when called on a
disabled counter, but mlxsw_sp_nexthop_counter_enable() can't similarly
be called on an enabled counter. This would be useful in the following
patches. Add the missing condition.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/0cc9050e196366c1387ab5ee47f1cee8ecde9c86.1709901020.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>