#7 UsbDevice needs abortAllSubmissions() for the DCP

[Dan Streetman]

There is no abortAllSubmissions() method on the
UsbDevice (i.e. DCP). The UsbPipe class does have an
abortAllSubmissons() method. The UsbDevice class
should have this method as well.

The USB spec should be checked to verify that this
method is required, i.e. that the spec actually states that
aborting all submissions on the DCP is mandatory (and
therefore theoretically always provided by the OS USB
subsystem).

[Dan Streetman]

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I am lowering the sev as I don't think the USB spec provides
for aborting a DCP pipe. Neither, I suspect, do most OS
implementations.