can: rcar_canfd: Describe channel-specific FD registers using C struct

can: rcar_canfd: Describe channel-specific FD registers using C struct

The rcar_canfd_f_*() inline functions to obtain channel-specific CAN-FD
register offsets really describe a memory layout.  Hence replace them by
a C structure, to simplify the code, and reduce kernel size.

This also gets rid of warnings about unused rcar_canfd_f_*() inline
functions, which are reported by recent versions of clang.

Suggested-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/20250618183827.5bebca8f@kernel.org
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://patch.msgid.link/292b75b3bc8dd95f805f0223f606737071c8cf86.1750327217.git.geert+renesas@glider.be
Signed-off-by: Jakub Kicinski <kuba@kernel.org>