tpm/tpm_crb: fix unused warnings on suspend/resume functions

When PM_SLEEP is disabled crb_pm_suspend and crb_pm_resume are not used by
SET_SYSTEM_SLEEP_PM_OPS even if PM is enabled:

drvers/char/tpm/tpm_crb.c:540:12: warning: ‘crb_pm_suspend’ defined but not
used [-Wunused-function]
 static int crb_pm_suspend(struct device *dev)
            ^
drivers/char/tpm/tpm_crb.c:551:12: warning: ‘crb_pm_resume’ defined but not
used [-Wunused-function]
 static int crb_pm_resume(struct device *dev)
            ^

The preprocessor condition should be on CONFIG_PM_SLEEP, not on CONFIG_PM.
However, this patch fixes this warning by using __maybe_unused on function
that are in the preprocessor condition.

Fixes: 848efcfb560c ("tpm/tpm_crb: enter the low power state upon device suspend")
Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Tested-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>