can: c_can: reg_map_{c,d}_can: mark as __maybe_unused

This patch marks the arrays reg_map_c_can and reg_map_d_can as __maybe_unused,
as they are indeed unused in the c_can driver. This warning shows up, when
compiling the kernel with "W=1":

    drivers/net/can/c_can/c_can.c:45:
    drivers/net/can/c_can/c_can.h:124:18: warning: ‘reg_map_d_can’ defined but not used [-Wunused-const-variable=]
    drivers/net/can/c_can/c_can.h:84:18: warning: ‘reg_map_c_can’ defined but not used [-Wunused-const-variable=]

Link: http://lore.kernel.org/r/20201006203748.1750156-4-mkl@pengutronix.de
Fixes: 33f81009 ("can: c_can: Move overlay structure to array with offset as index")
Fixes: 69927fcc

 ("can: c_can: Add support for Bosch D_CAN controller")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>