[media] smiapp: Make suspend and resume functions __maybe_unused

The smiapp_suspend() and smiapp_resume() functions will end up being unused
if CONFIG_PM is enabled but CONFIG_PM_SLEEP is disabled, causing a
compiler warning from both of the function definitions. Fix this by
marking the functions with __maybe_unused.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>