iio: hid-sensors: Use software trigger

Recent changes linux mainline resulted in warning:
"genirq: Warn about using IRQF_ONESHOT without a threaded handler"
when HID sensor hub is used.

When INDIO_BUFFER_TRIGGERED is used, the core attaches a poll function
when enabling the buffer. This poll function uses request_threaded_irq()
with both bottom half and top half handlers. But when using HID
sensor hub, bottom half (thread handler) is not registered.

In HID sensors, once a sensor is powered on, the hub collects samples
and pushes data to the host when programmed thresholds are met. When
this data is received for a sensor, it is pushed using
iio_push_to_buffers_with_ts().

The sensor is powered ON or OFF based on the trigger callback
set_trigger_state() when the poll function is attached. During the call
to iio_triggered_buffer_setup_ext(), the HID sensor specifies only a
handler function but provides no thread handler, as there is no data
to read from the hub in thread context. Internally, this results in
calling request_threaded_irq(). Recent kernel changes now warn when
request_threaded_irq() is called without a thread handler.

To address this issue, fundamental changes are required to avoid using
iio_triggered_buffer_setup_ext(). HID sensors can use
INDIO_BUFFER_SOFTWARE instead of INDIO_BUFFER_TRIGGERED, as this can
work in trigger-less mode.

In this approach, when user space opens the buffer, the sensor is powered
on, and when the buffer is closed, the sensor is powered off using
iio_buffer_setup_ops callbacks.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>