usb: host: xhci: introduce xhci_td_cleanup()

By extracting xhci_td_cleanup() from finish_td(), code before clearer
and easier to follow.

There are no functional changes with this patch. It's merely a cleanup.

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>