usb: host: xhci: combine event TRB completion debugging messages

If we just provide a helper to convert completion code to string, we can
combine all debugging messages into a single print.

[keep the old debug messages, for warn and grep -Mathias]
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>