ACPICA: use acpi_size instead of u32 for prefix_path_length

strlen() returns a size_t, so use acpi_size instead of u32 for
prefix_path_length.

ACPICA commit 0f64c317e769a63679442404421da1d5bd61068a

Link: https://github.com/acpica/acpica/commit/0f64c317


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>