usb: host: xhci: reorder variable definitions

no functional changes. Simple cleanup to make sure variables are ordered
in a 'reverse christmas tree' fashion. While at that, also remove an
obsolete comment which doesn't apply anymore.

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>