candev: add/update helpers for CAN FD

- update sanity checks
- add DLC to length conversion helpers
  - can_dlc2len() - get data length from can_dlc with sanitized can_dlc
  - can_len2dlc() - map the sanitized data length to an appropriate DLC

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>