can: mcp251xfd: silence clang warning

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.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>