x86_32: summit_32, use BAD_APICID

Use BAD_APICID instead of 0xFF constants in summit_cpu_mask_to_apicid.

Also remove bogus comments about what we actually return.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>