usb: dwc2: Fix build warning when CONFIG_PM_SLEEP=n
Building with bcm2835_defconfig, which has CONFIG_PM_SLEEP=n causes the
following build warning:
drivers/usb/dwc2/platform.c:227:12: warning: 'dwc2_suspend' defined but not used [-Wunused-function]
drivers/usb/dwc2/platform.c:237:12: warning: 'dwc2_resume' defined but not used [-Wunused-function]
Annotate these functions with '__maybe_unused' to prevent the warnings.