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>