xhci: Add missing annotation for xhci_enter_test_mode

Sparse reports a warning at xhci_enter_test_mode()

warning: context imbalance in  xhci_enter_test_mode - unexpected unlock

The root cause is the missing annotation at xhci_enter_test_mode()
Add the missing __must_hold(&xhci->lock) annotattion
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Link: https://lore.kernel.org/r/20200214204741.94112-24-jbi.octave@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-hub.c

@@ -609,6 +609,7 @@ static void xhci_port_set_test_mode(struct xhci_hcd *xhci,
 
 static int xhci_enter_test_mode(struct xhci_hcd *xhci,
 				u16 test_mode, u16 wIndex, unsigned long *flags)
+	__must_hold(&xhci->lock)
 {
 	int i, retval;