mem-cache: Initialize all members of `QueuedPrefetcher::DeferredPacket`.

mem-cache: Initialize all members of `QueuedPrefetcher::DeferredPacket`.

Members `tc` and `ongoingTranslation` were uninitialized in the constructor for
`QueuedPrefetcher::DeferredPacket`. If `ongoingTranslation` is not initialized to
`false` by default, some translation requests from queued prefetchers are not
properly handled and executions are nondeterministic.

Change-Id: Ia278f9e74847d6b847984d47f6a45643bae57794
Signed-off-by: Isaac Sánchez Barrera <isaac.sanchez@bsc.es>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22844
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>