staging:iio:sysfs-trigger: Use irq_work to properly active trigger

Since iio_trigger_poll() calls generic_handle_irq() it need to be called from
hardirq context. The sysfs trigger is kicked from userspace, so it is obviously
not possible to fulfill this requirement by calling iio_trigger_poll directly.
As a workaround commit 1f785681

 ("staging:iio:trigger sysfs userspace trigger
rework.") added iio_trigger_poll_chained() which uses handle_nested_irq instead
of generic_handle_irq. This in itself is a hack and only works by chance.
handle_nested_irq is intended to be called from the threaded interrupt handler
of the parent IRQ. Using handle_nested_irq is also problematic since it will
only call the threaded handler of the IRQ. But quite a few IIO drivers rely on
their hardirq handler being called or undefined behaviour will occur.

This patch uses the irq_work framework to schedule the call to
iio_trigger_poll() from hardirq context, which fixes the issues described above.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>