ASoC: fsl: remove unnecessary call to dma_unmap_single

Remove a call to dma_unmap_single() from the PowerPC ASoC DMA driver.  The
buffer is allocated and not actually mapped, so the unmap call doesn't
make sense.  It was probably left over from some early version of the driver.

This bug was unnoticed for so long because the DMA mapping functions normally
don't do anything on PowerPC.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>