When a umass storage device is unplugged there might be pending requests

When a umass storage device is unplugged there might be pending requests
which either fail or wind up in limbo.  The state machine will attempt to
issue a BBB RESET, but since this does not occur on our bulk or interrupt
pipes simply aborting the pipes is not sufficient to ensure that all of our
xfer requests have been idled.

We must also wait for any in-progress RESET's to complete, which is
accomplished by testing sc->transfer_state.  Otherwise we may wind up
freeing an active xfer request and blow up.