usb/xhci: fix compilation warning when !CONFIG_PCI && !CONFIG_PM

When CONFIG_PCI and CONFIG_PM are not selected, xhci.c gets this
warning:
drivers/usb/host/xhci.c:409:13: warning: ‘xhci_msix_sync_irqs’ defined
but not used [-Wunused-function]

Instead of creating nested #ifdefs, this patch fixes it by defining the
xHCI PCI stubs as inline.

Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>