can: m_can: mark runtime-PM handlers as __maybe_unused

Building without CONFIG_PM results in a harmless warning:

drivers/net/can/m_can/m_can.c:1763:12: error: 'm_can_runtime_resume' defined but not used [-Werror=unused-function]
drivers/net/can/m_can/m_can.c:1752:12: error: 'm_can_runtime_suspend' defined but not used [-Werror=unused-function]

Marking the functions as __maybe_unused lets the compiler
silently drop them instead.

Fixes: cdf8259d ("can: m_can: Add PM Support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>