ACPI/ERST: fix signed/unsigned type conflicts

ACPI/ERST: fix signed/unsigned type conflicts

Error checks exist in the respective code path that expect negative
values to indicate errors, yet negative values can't be communicated
through size_t. Thus ssize_t needs to be introduced (also on ARM for
consistency, even if the code in question isn't currently being used
on there).

The bug is theoretical only in so far as all the involved code is
effectively dead. Reflect this by excluding that code from non-debug
builds.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Christoph Egger <chegger@amazon.de>