usb: renesas_usbhs: mark PM functions as __maybe_unused
Without CONFIG_PM, we get a new build warning here:
drivers/usb/renesas_usbhs/common.c:860:12: error: 'usbhsc_resume' defined but not used [-Werror=unused-function]
static int usbhsc_resume(struct device *dev)
^~~~~~~~~~~~~
drivers/usb/renesas_usbhs/common.c:844:12: error: 'usbhsc_suspend' defined but not used [-Werror=unused-function]
static int usbhsc_suspend(struct device *dev)
^~~~~~~~~~~~~~