Most transfer events have a TRB pointer indicating which TRB caused
the event.
In the case of streams, transfer events such as
USB Transaction error may have its TRB pointer set to zero.

driver won't know which stream or what TRB on that stream caused
the error, but it can issue a soft reset to recover the transfer.
A soft reset will clear the host side halt of the endpoint without
clearing Data toggle or sequence number, and let the transfer
continue from where it halted.

see xhci section 4.12 streams and 4.6.8.2 soft retry.

USB Transaction errors with a zero TRB pointer are seen with
UAS usb devices.
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>