ACPICA: cast the result of the pointer difference to u32

Altnatively we could declare aml_length as acpi_size, but then one
would need to cast at the assignment for method_obj->method.aml_length

ACPICA commit 72805936603fcf84e98f1b89bf99b5101af27fb8

Link: https://github.com/acpica/acpica/commit/72805936


Signed-off-by: Sven Barth <sb@miray.de>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Erik Kaneda <erik.kaneda@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>