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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>