dmaengine: imx-sdma: return sdmac->status in sdma_tx_status()

The sdmac->status was designed to reflect the status of the tx,
so simply return it in sdma_tx_status().  Then dma client can call
dma_async_is_tx_complete() to know the status of the tx.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>