usb: xhci: lock mutex on xhci_stop

Else it races with xhci_setup_device

Cc: <stable@vger.kernel.org>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Change-Id: I00775500870968e444e32ae68b736dc73c44435d
Git-commit: 85ac90f8953a58f6a057b727bc9db97721e3fb8e
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Jack Pham <jackp@codeaurora.org>