usb: dwc3: gadget: Support Multi-Stream Transfer

Synopsys introduced a new enhancement to DWC_usb32 called Multi-Stream
Transfer (MST) to improve bulk streams performance for SuperSpeed and
SuperSpeed Plus. This enhancement allows the controller to look ahead
and process multiple bulk streams.

Previously, to initiate a bulk stream transfer, the driver has to issue
Start Transfer command and wait for the stream to complete before
initiating a new stream. As a result, the controller does not process
TRBs beyond a single stream. With the enhancement, as long as there are
new requests, the dwc3 driver can keep preparing new TRBs and the
controller can keep caching and processing them without waiting for the
transfer completion.

The programming flow is similar to regular bulk endpoint with a few
additional rules:

1) Chained TRBs of the same stream must have a matching stream ID
2) The last TRB of a stream must have CHN=0
3) All the TRBs with LST=0 must have CSP=1

Depends on the application and usage, internal tests show significant
performance improvement in UASP transfers with this enhancement.

Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/cd9c7a8bf11f790983ac546222dd114893f16b3a.1638242424.git.Thinh.Nguyen@synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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