dmaengine: imx-sdma: remove IMX_DMA_SG_LOOP handling in sdma_prep_slave_sg()

This is a leftover from the time that the driver did not have
sdma_prep_dma_cyclic callback and implemented sound dma as a looped
sg chain.  And it can be removed now.

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