usb: host: xhci: extract xhci_slot_state_string()

By extracting and exposing xhci_slot_state_string() in a header file, we
can re-use it to print Slot Context State from our tracepoints, which
can aid in tracking down problems related to command execution.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>