can: canxl: support Remote Request Substitution bit access

The Remote Request Substitution bit is a dominant bit ("0") in the CAN
XL frame. As some CAN XL controllers support to access this bit a new
CANXL_RRS value has been defined for the canxl_frame.flags element.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20250124142347.7444-1-socketcan@hartkopp.net
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

include/uapi/linux/can.h

@@ -182,7 +182,7 @@ struct canfd_frame {
 /*
  * defined bits for canxl_frame.flags
  *
- * The canxl_frame.flags element contains two bits CANXL_XLF and CANXL_SEC
+ * The canxl_frame.flags element contains three bits CANXL_[XLF|SEC|RRS]
  * and shares the relative position of the struct can[fd]_frame.len element.
  * The CANXL_XLF bit ALWAYS needs to be set to indicate a valid CAN XL frame.
  * As a side effect setting this bit intentionally breaks the length checks
@@ -192,6 +192,7 @@ struct canfd_frame {
  */
 #define CANXL_XLF 0x80 /* mandatory CAN XL frame flag (must always be set!) */
 #define CANXL_SEC 0x01 /* Simple Extended Content (security/segmentation) */
+#define CANXL_RRS 0x02 /* Remote Request Substitution */
 
 /* the 8-bit VCID is optionally placed in the canxl_frame.prio element */
 #define CANXL_VCID_OFFSET 16 /* bit offset of VCID in prio element */