can: rename CAN FD related can_len2dlc and can_dlc2len helpers

can: rename CAN FD related can_len2dlc and can_dlc2len helpers

The helper functions can_len2dlc and can_dlc2len are only relevant for
CAN FD data length code (DLC) conversion.

To fit the introduced can_cc_dlc2len for Classical CAN we rename:

can_dlc2len -> can_fd_dlc2len to get the payload length from the DLC
can_len2dlc -> can_fd_len2dlc to get the DLC from the payload length

Suggested-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/r/20201110101852.1973-6-socketcan@hartkopp.net
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>