ASoC: SOF: imx: make imx8m_dsp_ops static

This was in the SOF tree but lost in upstream contributions.

Fixes: afb93d71

 ("ASoC: SOF: imx: Add i.MX8M HW support")
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Link: https://lore.kernel.org/r/20200515135958.17511-9-kai.vehmanen@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>