usb: xhci: Expose xhci_start() function.

Change the visability of xhci_start() so that it
can be used when enabling test mode.

Signed-off-by: Guoqing Zhang <guoqing.zhang@intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>