doc: reference: canbus: use DEVICE_DT_GET for the samples

Use DEVICE_DT_GET instead of device_get_binding on the samples. Also,
use the zephyr,canbus chosen device. Note that check for readiness has
not been added as previous code did not check for NULL either.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>