usb: dwc3: qcom: mark PM functions as __maybe_unused

The #ifdef guards around these are wrong, resulting in warnings
in certain configurations:

drivers/usb/dwc3/dwc3-qcom.c:244:12: error: 'dwc3_qcom_resume' defined but not used [-Werror=unused-function]
 static int dwc3_qcom_resume(struct dwc3_qcom *qcom)
            ^~~~~~~~~~~~~~~~
drivers/usb/dwc3/dwc3-qcom.c:223:12: error: 'dwc3_qcom_suspend' defined but not used [-Werror=unused-function]
 static int dwc3_qcom_suspend(struct dwc3_qcom *qcom)

This replaces the guards with __maybe_unused annotations to shut up
the warnings and give better compile time coverage.

Fixes: a4333c3a

 ("usb: dwc3: Add Qualcomm DWC3 glue driver")
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>