usb: mtu3: fix the failure of qmu stop

This happens when do stress test of uvc stream on/off which will
enable/disable endpoints. uvc has four tx requests, and may disable
endpoint between queue tx requests as following:
    enable ep --> start qmu
    queue tx request0
    queue tx request1
    queue tx request2 --> resume qmu
    disable ep --> stop qmu may fail [1]
    queue tx request3 --> will resume qmu, may cause qmu can't work
                          when enable ep next time [2]

[1]: when the tx fifo has some data to transmit, and
    try to stop qmu (stop ep) meanwhile resume qmu (queue tx request),
    it may cause stop qmu timeout, then can be fixed by flushing fifo
    when stop qmu.
[2]: it resumes qmu again, shall stop qmu again.

Signed-off-by: Chunfeng Yun <[email protected]>
Reported-by: Min Guo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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