can: rcar_canfd: Repurpose f_dcfg base for other registers

Reuse the existing Channel Data Bitrate Configuration Register offset
member in the register configuration as the base offset for all related
channel-specific registers.
Rename the member and update the (incorrect) comment to reflect this.
Replace the function-like channel-specific register offset macros by
inline functions.

This fixes the offsets of all other (currently unused) channel-specific
registers on R-Car Gen4 and RZ/G3E, and allows us to replace
RCANFD_GEN4_FDCFG() by the more generic rcar_canfd_f_cfdcfg().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://patch.msgid.link/75c8197c849fc9e360a75d4fa55bc01c1d850433.1749655315.git.geert+renesas@glider.be


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