iio: imu: st_lsm6dsx: fix iio_chan_spec for sensors without event detection

iio: imu: st_lsm6dsx: fix iio_chan_spec for sensors without event detection

The st_lsm6dsx_acc_channels array of struct iio_chan_spec has a non-NULL
event_spec field, indicating support for IIO events. However, event
detection is not supported for all sensors, and if userspace tries to
configure accelerometer wakeup events on a sensor device that does not
support them (e.g. LSM6DS0), st_lsm6dsx_write_event() dereferences a NULL
pointer when trying to write to the wakeup register.
Define an additional struct iio_chan_spec array whose members have a NULL
event_spec field, and use this array instead of st_lsm6dsx_acc_channels for
sensors without event detection capability.

Fixes: b5969abfa8b8 ("iio: imu: st_lsm6dsx: add motion events")
Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>