[siba] fix up debugging.

• unbreak non-debug builds - don't default to debugging SCAN; that was left-over from my testing.
• include opt_siba.h, now that it's generated as appropriate.
• stick the debug enum outside the debug block, just so it's there for any code that wants to set siba_debug for some reason (like say, my debugging muckup.)
• make DPRINTF() use VA_ARGS for formatting too, so it correctly handles printing w/ no args.
• Make DPRINTF() use device_printf().

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