Staging: iio: Move evgen interrupt generation to irq_work

Enhance interrupt generation in the dummy driver and expand its usage
by introducing the irq_work infrastructure to trigger an interrupt.

This way, the irq_work_queue() wrapper permits calling both of the top
half and threaded part from a hard irq context, unlike handle_nested_irq(),
which only calls the threaded part.

As an outcome, the driver succeeds in simulating real hardware
interrupts, while keeping the normal interrupt flow.

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>