ASoC: imx: remove superfluous code in imx-ssi.c

Checking if IMX_SSI_DMA is set and then set it again is useless.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>