can: at91_can: silence clang warning

This patch fixes the following clang warning, by marking the functions
as maybe unused. gcc doesn't complain about unused inline functions.

| drivers/net/can/at91_can.c:178:1: warning: unused function 'at91_is_sam9X5' [-Wunused-function]
| AT91_IS(9X5);
| ^
| drivers/net/can/at91_can.c:172:19: note: expanded from macro 'AT91_IS'
| static inline int at91_is_sam##_model(const struct at91_priv *priv) \
|                   ^
| <scratch space>:66:1: note: expanded from here
| at91_is_sam9X5
| ^

Link: https://lore.kernel.org/r/20210514153741.1958041-2-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/at91_can.c

@@ -169,7 +169,7 @@ static const struct can_bittiming_const at91_bittiming_const = {
 };
 
 #define AT91_IS(_model) \
-static inline int at91_is_sam##_model(const struct at91_priv *priv) \
+static inline int __maybe_unused at91_is_sam##_model(const struct at91_priv *priv) \
 { \
 	return priv->devtype_data.type == AT91_DEVTYPE_SAM##_model; \
 }