usb: xhci-mtk: use __maybe_unused to hide pm functions

The mediatek XHCI glue driver uses SET_SYSTEM_SLEEP_PM_OPS() to
conditionally set the correct suspend/resume options, and
also puts both the dev_pm_ops and the functions inside of
an #ifdef testing for CONFIG_PM_SLEEP, but those functions
then call other code that becomes unused:

drivers/usb/host/xhci-mtk.c:135:12: error: 'xhci_mtk_host_disable' defined but not used [-Werror=unused-function]
drivers/usb/host/xhci-mtk.c:313:13: error: 'usb_wakeup_enable' defined but not used [-Werror=unused-function]
drivers/usb/host/xhci-mtk.c:321:13: error: 'usb_wakeup_disable' defined but not used [-Werror=unused-function]

This replaces the #ifdef with __maybe_unused annotations so the
compiler knows it can silently drop them instead of warning.

For the DEV_PM_OPS definition, we can use an IS_ENABLED() check
to avoid defining the structure when CONFIG_PM is not set without
the #ifdef.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>