devices: vfio: remove unnecessary MemSlot casts
devices: vfio: remove unnecessary MemSlot casts

The values that were cast in these expressions were already MemSlot, so
there is no need to cast back and forth between MemSlot and u32.

BUG=None
TEST=tools/presubmit

Change-Id: I8a215bc50d10a93b21f6d2fe7e4ed7267242a50b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3508320
Reviewed-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm