dmaengine: sh: rcar-dmac: Fix glitch in dmaengine_tx_status

The tx_status poll in the rcar_dmac driver reads the status register
which indicates which chunk is busy (DMACHCRB). Afterwards the point
inside the chunk is read from DMATCRB. It is possible that the chunk
has changed between the two reads. The result is a non-monotonous
increase of the residue. Fix this by introducing a 'safe read' logic.

Fixes: 73a47bd0

 ("dmaengine: rcar-dmac: use TCRB instead of TCR for residue")
Signed-off-by: Achim Dahlhoff <Achim.Dahlhoff@de.bosch.com>
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Cc: <stable@vger.kernel.org> # v4.16+
Signed-off-by: Vinod Koul <vkoul@kernel.org>