xhci: Add missing annotation for xhci_set_port_power()

Sparse reports a warning at xhci_set_port_power()

warning: context imbalance in xhci_set_port_power - unexpected unlock

The root cause is the missing annotation at xhci_set_port_power()
Add the missing __must_hold(&xhci->lock) annotattion
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Link: https://lore.kernel.org/r/20200214204741.94112-23-jbi.octave@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-hub.c

@@ -558,6 +558,7 @@ struct xhci_hub *xhci_get_rhub(struct usb_hcd *hcd)
  */
 static void xhci_set_port_power(struct xhci_hcd *xhci, struct usb_hcd *hcd,
 				u16 index, bool on, unsigned long *flags)
+	__must_hold(&xhci->lock)
 {
 	struct xhci_hub *rhub;
 	struct xhci_port *port;