ASoC: imx: fix burstsize for DMA

SSI counts in words, the DMA engine in bytes. (Wrong) factor got removed
in bf974a0d (ASoC i.MX: switch to new DMA api).

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>
Cc: stable@kernel.org