[SCSI] fix usb storage oops

The problem is that scsi_run_queue is called from scsi_next_command()
after doing a scsi_put_command.  If the command was the only thing
holding the reference on the scsi_device then the resulting device put
will tear down the block queue.  Fix this by taking a reference to the
device and holding it around scsi_run_queue()

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>