ACPI / APEI: remove redundant variables len and node_len

Variables len and node_len are redundant and can be removed. Cleans
up clang warning: node_len = GHES_ESTATUS_NODE_LEN(len);

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>