usb: host: xhci: remove #ifdef around PM functions

The #ifdef is slightly wrong as it doesn't cover the xhci_priv_resume_quirk()
function, causing a harmless warning:

drivers/usb/host/xhci-plat.c:58:12: error: 'xhci_priv_resume_quirk' defined but not used [-Werror=unused-function]
 static int xhci_priv_resume_quirk(struct usb_hcd *hcd)

A simpler way to do this correctly is to use __maybe_unused annotations
that let the compiler silently drop the functions when there is no
reference.

Fixes: b0c69b4b

 ("usb: host: plat: Enable xHCI plat runtime PM")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>