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 <luis.hernandez093@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Reviewed-by: Pavel Pisa <pisa@fel.cvut.cz>
Link: https://patch.msgid.link/20250722035352.21807-1-luis.hernandez093@gmail.com


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>