drivers: sensor: adxl355: fix is_fifo offset mismatch in decoder routing

adxl355_decoder_decode() reinterprets a raw sample buffer as
struct adxl355_fifo_data first, checking is_fifo at byte offset 0 to
decide whether to route to adxl355_decode_stream(). struct
adxl355_sample declared is_fifo last (after x/y/z/range), so offset 0
of a one-shot sample buffer was actually the low byte of x - real
accelerometer data, not a flag.

With CONFIG_ADXL355_STREAM=y, roughly half of all one-shot reads
(whenever bit 0 of the raw X LSB happened to be set) were misrouted
into adxl355_decode_stream(), which then reads fifo_byte_count and
sample_set_size at offsets 20-22 - past the 20-byte (sizeof(struct
adxl355_sample)) buffer - causing an out-of-bounds read.

Fix this by giving adxl355_sample an is_fifo field at offset 0,
matching adxl355_fifo_data's layout, and have adxl355_read_sample()
explicitly clear it rather than relying on zero-initialization.

Signed-off-by: Dimitrije Lilic <dimitrije.lilic@orioninc.com>

Author: dimitrije-lilic

drivers/sensor/adi/adxl355/adxl355.c

@@ -623,6 +623,7 @@ static int adxl355_read_sample(const struct device *dev)
 	sample->y = combine_bytes_to_int20(buf[3], buf[4], buf[5]);
 	sample->z = combine_bytes_to_int20(buf[6], buf[7], buf[8]);
 	sample->range = data->range;
+	sample->is_fifo = 0;
 	return 0;
 }
 

drivers/sensor/adi/adxl355/adxl355.h

@@ -269,11 +269,11 @@ struct adxl355_extra_attr {
  *
  */
 struct adxl355_sample {
+	uint8_t is_fifo: 1;
 	int32_t x;
 	int32_t y;
 	int32_t z;
 	enum adxl355_range range;
-	uint8_t is_fifo: 1;
 };
 
 /**