usb: xhci: Fix forbidden warning for unused function.

The warning 'xhci_msix_sync_irqs' defined but not used happened in case
both CONFIG_PCI and CONFIG_PM are not set.

Change-Id: Id94b02fd2d2a87e5df79d32ff251e2985db22554
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>