mlxsw: spectrum_trap: Do not hard code "thin" policer identifier

As explained in commit e6125230 ("mlxsw: spectrum_trap: Introduce
dummy group with thin policer"), the purpose of the "thin" policer is to
pass as less packets as possible to the CPU.

The identifier of this policer is currently set according to the maximum
number of used trap groups, but this is fragile: On Spectrum-1 the
maximum number of policers is less than the maximum number of trap
groups, which might result in an invalid policer identifier in case the
number of used trap groups grows beyond the policer limit.

Solve this by dynamically allocating the policer identifier.

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