[UPSTREAM] Attempt at improving ADC access

[Andy Green]

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Somebody in the thread at some point said:

| Also, don't the queue head/tail manipulation functions require locking ?

When I wrote them I recall coming to the idea that no locking is needed
because each of the "threads" only controls the head or the tail
exclusively.

| +		req = pcf->adc_queue[tail];
| +		WARN_ON(!pcf);

You might as well either BUG_ON() before it or just let it blow up on
NULL, it would blow up with a NULL OOPS even if the warning fired before.

- -Andy

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