can: dev: fix type of get_can_dlc() and get_canfd_dlc() macros

The macros get_can_dlc() and get_canfd_dlc() are not visible in
userland. As such, type u8 should be preferred over type __u8.

Reference: https://lkml.org/lkml/2020/10/1/708
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/r/20201002154219.4887-3-mailhol.vincent@wanadoo.fr
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>