docs: Fix kernel-doc error in CAN driver

Fix kernel-doc formatting issue causing unexpected indentation error
in ctucanfd driver documentation build. Convert main return values
to bullet list format while preserving numbered sub-list in order to
correct indentation error and visual structure in rendered html.

Signed-off-by: Luis Felipe Hernandez <[e-mail]>
Reviewed-by: Randy Dunlap <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Reviewed-by: Vincent Mailhol <[e-mail]>
Reviewed-by: Pavel Pisa <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

drivers/net/can/ctucanfd/ctucanfd_base.c

···
  * @buf:	TXT Buffer index to which frame is inserted (0-based)
  * @isfdf:	True - CAN FD Frame, False - CAN 2.0 Frame
  *
- * Return: True - Frame inserted successfully
- *	   False - Frame was not inserted due to one of:
- *			1. TXT Buffer is not writable (it is in wrong state)
- *			2. Invalid TXT buffer index
- *			3. Invalid frame length
+ * Return:
+ * * True - Frame inserted successfully
+ * * False - Frame was not inserted due to one of:
+ *	1. TXT Buffer is not writable (it is in wrong state)
+ *	2. Invalid TXT buffer index
+ *	3. Invalid frame length
  */
 static bool ctucan_insert_frame(struct ctucan_priv *priv, const struct canfd_frame *cf, u8 buf,
 				bool isfdf)