iio: adc: max1027: Use the EOC IRQ when populated for single reads

iio: adc: max1027: Use the EOC IRQ when populated for single reads

So far the End-Of-Conversion interrupt was only used in conjunction with
the internal trigger to process the data. Let's extend the use of this
interrupt handler to support regular single-shot conversions as well.

Doing so requires writing our own hard IRQ handler. This handler has to
check if buffers are enabled or not:

*** Buffers disabled condition ***

  This means the user requested a single conversion and the sample is
  ready to be retrieved.

    -> This implies adding the relevant completion boilerplate.

*** Buffers enabled condition ***

  Triggers are used. So far there is only support for the internal
  trigger but this trigger might soon be attached to another device as
  well so it is the core duty to decide which handler to call in order
  to process the data. The core will decide to either:

  * Call the internal trigger handler which will extract the data that
    is already present in the ADC FIFOs

  or

  * Call the trigger handler of another driver when using this trigger
    with another device, even though this call will be slightly delayed
    by the fact that the max1027 IRQ is a data-ready interrupt rather
    than a real trigger:

  -> The new handler will manually inform the core about the trigger
     having transitioned by directly calling iio_trigger_poll() (which
     iio_trigger_generic_data_rdy_poll() initially did).

In order for the handler to be "source" agnostic, we also need to change
the private pointer and provide the IIO device instead of the trigger
object.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20210921115408.66711-15-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>