selftests: mlxsw: resource_scale: Update scale target after test setup

The scale of each resource is tested in the following manner:

1. The scale target is queried.
2. The test setup is prepared.
3. The test is invoked.

In some cases, the occupancy of a resource changes as part of the second
step, requiring the test to return a scale target that takes this change
into account.

Make this more robust by re-querying the scale target after the second
step.

Another possible solution is to swap the first and second steps, but
when a test needs to be skipped (i.e., scale target is zero), the setup
would have been in vain.

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