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

commit 01bb59ebffdec314da8da66266edf29529372f9b upstream.

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.

This warning has been in since 3.2 kernel and was
caused by commit 421aa841
"usb/xhci: hide MSI code behind PCI bars", but wasn't noticed
until 3.13 when a configuration with these options was tried

Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>