usb: xhci: Mark two functions __maybe_unused

Resolves the following build warnings:
drivers/usb/host/xhci.c:332:13: warning: 'xhci_msix_sync_irqs' defined but not used [-Wunused-function]
drivers/usb/host/xhci.c:3901:12: warning: 'xhci_change_max_exit_latency' defined but not used [-Wunused-function]

These functions are not always used, and since they're marked static
they will produce build warnings:
- xhci_msix_sync_irqs is only used with CONFIG_PCI.
- xhci_change_max_exit_latency is a little more complicated with
  dependencies on CONFIG_PM and CONFIG_PM_RUNTIME.

Instead of building a bigger maze of ifdefs in this code, I've just
marked both with __maybe_unused.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>