can: length: can_fd_len2dlc(): make legnth calculation readable again

can: length: can_fd_len2dlc(): make legnth calculation readable again

In commit 0451285d9559 ("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: 0451285d9559 ("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.net
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>