can: mcp251xfd: 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/spi/mcp251xfd/mcp251xfd-core.c:564:1: warning: unused function 'mcp251xfd_chip_set_mode_nowait' [-Wunused-function]
| mcp251xfd_chip_set_mode_nowait(const struct mcp251xfd_priv *priv,
| ^
| 1 warning generated.

Link: https://lore.kernel.org/r/20210514153741.1958041-3-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c

@@ -560,7 +560,7 @@ mcp251xfd_chip_set_mode(const struct mcp251xfd_priv *priv,
 	return __mcp251xfd_chip_set_mode(priv, mode_req, false);
 }
 
-static inline int
+static inline int __maybe_unused
 mcp251xfd_chip_set_mode_nowait(const struct mcp251xfd_priv *priv,
 			       const u8 mode_req)
 {