[UPSTREAM] Attempt at improving ADC access

[Werner Almesberger]

Balaji Rao wrote:
> Here's an attempt to improve ADC access in the pcf50633 driver.
> I've introduced a method for synchronously reading from the ADC.

Very nice work ! One suggestion for a slight simplification: you can
avoid distinguishing synchronous and asynchronous by using a callback
in adc_sync_read that does the completion.

- Werner