In commit 652562e5 ("can: length: can_fd_len2dlc(): simplify length
calculcation") the readability of the code degraded and became more error
prone. To counteract this, partially convert that patch and replace open coded
values (of the original code) with proper defines.

Fixes: 652562e5 ("can: length: can_fd_len2dlc(): simplify length calculcation")
Cc: Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/r/20210118201346.79422-1-socketcan@hartkopp.netSigned-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>