selftests: mlxsw: Reduce test run time

Instead of iterating over all the available trap policers, only perform
the tests with three policers: The first, the last and the one in the
middle of the range. On a Spectrum-3 system, this reduces the run time
from almost an hour to a few minutes.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>