uas: keep track of command state, finish scsi cmd when really done.

Set state bits after submitting data urbs & command urbs, so we know
what is in flight.  Clear data bits when the data urb is finished, clear
command bit when we see the status urb for the command.  Finish the scsi
command after running both status and data completion handlers for the
command.

Add a cmd status logging function for debugging purposes.  Hook it into
the error handler, so we see in the log what status a command is in
which the scsi layer wants cancel.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>