staging: precedence bug in crystalhd_stop_tx_dma_engine()

The intent here is to see if we have cleared the DMA_START_BIT flag.  We
clear it a couple lines later.  The current code has a precedence bug so
it is equivalent to "if (!dma_cntrl) { ...".

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>