dmaengine: Add wrapper for device_tx_status callback

This patch adds a small inline wrapper for the devivce_tx_status callback of a
dma device. This makes the source code of users of this function a bit more
compact and a bit more legible.

E.g.:
-status = chan->device->device_tx_status(chan, cookie, &state)
+status = dmaengine_tx_status(chan, cookie, &state)

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>