base: Fix storage params safe_cast

base: Fix storage params safe_cast

Although they provide the exact same behavior, the params
created in the tests did not have the type expected by the
internal safe cast.

The following error was triggered:

storage.test.debug: build/NULL/base/cast.hh:47: T safe_cast(U)
 [with T = const Stats::SampleStor::Params*;
  U = const Stats::StorageParams*]:
 Assertion `ret' failed.

Change-Id: I4f2ba51f3ccdb44589e61f235997245e7d9bf3c9
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40555
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>