can: rcar_canfd: Simplify data access in rcar_canfd_{ge,pu}t_data()

Replace the repeated casts, pointer additions, and pointer dereferences
by array accesses to improve readability.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://patch.msgid.link/4f43f44dcfda13d48a2c502648833934a51d9d6c.1749655315.git.geert+renesas@glider.be


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