slimbus: ngd: mark PM functions as __maybe_unused

qcom_slim_ngd_runtime_suspend is protected by an #ifdef,
qcom_slim_ngd_runtime_idle is now, which causes a build time warning:

drivers/slimbus/qcom-ngd-ctrl.c:1470:12: error: 'qcom_slim_ngd_runtime_idle' defined but not used [-Werror=unused-function]

Marking both as __maybe_unused lets us get rid of the warning
as well as the #ifdef.

Fixes: 917809e2 ("slimbus: ngd: Add qcom SLIMBus NGD driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>