iio: imu: st_lsm6dsx: rework code to check for enabled events

iio: imu: st_lsm6dsx: rework code to check for enabled events

The enable_event field in struct st_lsm6dsx_hw does not lend itself well to
handling multiple event sources, so it will have to be modified to add
support for more event sources. As a preparatory step, remove references to
this field from code that does not deal with event management; rework the
st_lsm6dsx_check_events() function so that it returns whether any events
are currently enabled on a given sensor.

Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>