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 <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Acked-by: Lorenzo Bianconi <[e-mail]>
Cc: [e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c

···
 	IIO_CHAN_SOFT_TIMESTAMP(3),
 };
 
+static const struct iio_chan_spec st_lsm6ds0_acc_channels[] = {
+	ST_LSM6DSX_CHANNEL(IIO_ACCEL, 0x28, IIO_MOD_X, 0),
+	ST_LSM6DSX_CHANNEL(IIO_ACCEL, 0x2a, IIO_MOD_Y, 1),
+	ST_LSM6DSX_CHANNEL(IIO_ACCEL, 0x2c, IIO_MOD_Z, 2),
+	IIO_CHAN_SOFT_TIMESTAMP(3),
+};
+
 static const struct iio_chan_spec st_lsm6dsx_gyro_channels[] = {
 	ST_LSM6DSX_CHANNEL(IIO_ANGL_VEL, 0x22, IIO_MOD_X, 0),
 	ST_LSM6DSX_CHANNEL(IIO_ANGL_VEL, 0x24, IIO_MOD_Y, 1),
···
 		},
 		.channels = {
 			[ST_LSM6DSX_ID_ACC] = {
-				.chan = st_lsm6dsx_acc_channels,
-				.len = ARRAY_SIZE(st_lsm6dsx_acc_channels),
+				.chan = st_lsm6ds0_acc_channels,
+				.len = ARRAY_SIZE(st_lsm6ds0_acc_channels),
 			},
 			[ST_LSM6DSX_ID_GYRO] = {
 				.chan = st_lsm6ds0_gyro_channels,
···
 		},
 		.channels = {
 			[ST_LSM6DSX_ID_ACC] = {
-				.chan = st_lsm6dsx_acc_channels,
-				.len = ARRAY_SIZE(st_lsm6dsx_acc_channels),
+				.chan = st_lsm6ds0_acc_channels,
+				.len = ARRAY_SIZE(st_lsm6ds0_acc_channels),
 			},
 			[ST_LSM6DSX_ID_GYRO] = {
 				.chan = st_lsm6dsx_gyro_channels,