sensor: bmi160: Fix address-of-packed-mem warning

The warning below appears once -Waddress-of-packed-mem is enabled:

/__w/zephyr/zephyr/drivers/sensor/bmi160/bmi160.c: In function
bmi160_gyr_channel_get:
/__w/zephyr/zephyr/drivers/sensor/bmi160/bmi160.c:795:23: error: taking
address of packed member of struct <anonymous> may result in an
unaligned pointer value [-Werror=address-of-packed-member]
  795 |           data->sample.gyr, val);
      |           ~~~~~~~~~~~~^~~~
/__w/zephyr/zephyr/drivers/sensor/bmi160/bmi160.c: In function
bmi160_acc_channel_get:
/__w/zephyr/zephyr/drivers/sensor/bmi160/bmi160.c:807:23: error: taking
address of packed member of struct <anonymous> may result in an
unaligned pointer value [-Werror=address-of-packed-member]
  807 |           data->sample.acc, val);
      |           ~~~~~~~~~~~~^~~~

To avoid the warning, make the struct non-packed, since it is not
necessary in this case given that a union already guarantees that the
pointer to the union points to each member of the union equally..

More info in #16587.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>