ASoC: omap: sdma-pcm: Fix modpost warning

WARNING: modpost: missing MODULE_LICENSE() in sound/soc/omap/snd-soc-sdma.o
see include/linux/module.h for more information
WARNING: modpost: missing MODULE_LICENSE() in sound/soc/omap/snd-soc-sdma.o
see include/linux/module.h for more information

Add the missing MODULE_LICENSE.

This patch also going to solve:
snd_soc_sdma: Unknown symbol devm_kmalloc (err 0)
snd_soc_sdma: Unknown symbol omap_dma_filter_fn (err 0)
snd_soc_sdma: Unknown symbol snd_dmaengine_pcm_prepare_slave_config (err 0)
snd_soc_sdma: Unknown symbol devm_snd_dmaengine_pcm_register (err 0)

Reported-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>