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

In the Linux kernel, the following vulnerability has been resolved:

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.