can: rcar_canfd: add __maybe_unused annotation to silence warning

Since commit

| dd3bd23e ("can: rcar_canfd: Add Renesas R-Car CAN FD driver")

the rcar_canfd driver can be compile tested on all architectures. On
non OF enabled archs, or archs where OF is optional (and disabled in
the .config) the compilation throws the following warning:

| drivers/net/can/rcar/rcar_canfd.c:2020:34: warning: unused variable 'rcar_canfd_of_table' [-Wunused-const-variable]
| static const struct of_device_id rcar_canfd_of_table[] = {
|                                  ^

This patch fixes the warning by marking the variable
rcar_canfd_of_table as __maybe_unused.

Fixes: ac422408 ("can: rcar: Kconfig: Add helper dependency on COMPILE_TEST")
Fixes: dd3bd23e

 ("can: rcar_canfd: Add Renesas R-Car CAN FD driver")
Link: https://lore.kernel.org/all/20210907064537.1054268-1-mkl@pengutronix.de
Cc: linux-renesas-soc@vger.kernel.org
Cc: Cai Huoqing <caihuoqing@baidu.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>