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

In commit 652562e5ff ("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: 652562e5ff ("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>

drivers/net/can/dev/length.c

@@ -27,12 +27,17 @@ static const u8 len2dlc[] = {
 	13, 13, 13, 13, 13, 13, 13, 13,	/* 25 - 32 */
 	14, 14, 14, 14, 14, 14, 14, 14,	/* 33 - 40 */
 	14, 14, 14, 14, 14, 14, 14, 14,	/* 41 - 48 */
+	15, 15, 15, 15, 15, 15, 15, 15,	/* 49 - 56 */
+	15, 15, 15, 15, 15, 15, 15, 15	/* 57 - 64 */
 };
 
 /* map the sanitized data length to an appropriate data length code */
 u8 can_fd_len2dlc(u8 len)
 {
-	if (len >= ARRAY_SIZE(len2dlc))
+	/* check for length mapping table size at build time */
+	BUILD_BUG_ON(ARRAY_SIZE(len2dlc) != CANFD_MAX_DLEN + 1);
+
+	if (unlikely(len > CANFD_MAX_DLEN))
 		return CANFD_MAX_DLC;
 
 	return len2dlc[len];