staging:iio:cdc:ad7150: Rework interrupt handling.

Note this doesn't support everything the chip can do as we ignore
window mode for now (in window / out of window).

* Given the chip doesn't have any way of disabling the threshold
  pins, use disable_irq() etc to mask them except when we actually
  want them enabled (previously events were always enabled).
  Note there are race conditions, but using the current state from
  the status register and disabling interrupts across changes in
  type of event should mean those races result in interrupts,
  but no events to userspace.

* Correctly reflect that there is one threshold line per channel.

* Only take notice of rising edge. If anyone wants the other edge
  then they should set the other threshold (they are available for
  rising and falling directions).  This isn't perfect but it makes
  it a lot simpler.

* If insufficient interrupts are specified in firnware, don't support
  events.

* Adaptive events use the same pos/neg values of thrMD as non adaptive
  ones.

Tested against qemu based emulation.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210314181511.531414-13-jic23@kernel.org

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