dmaengine: ti-dma-crossbar: Fix dra7 reserve function

DMA crossbar uses 'xbar->dma_inuse' variable to manage allocated routes.
Each bit represents respective DMA channel. If the channel is free, bit
is set to '0', if channel is allocated, bit should be set to '1'.

In reserve function, the bits for requested DMA channels are cleared, so
they are not really reserved, but freed and become ready for allocation.

Signed-off-by: Alexander Smirnov <asmirnov@ilbers.de>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>