ASoC: SOF: Mark get_ext* function ext_hdr arguments as const

ASoC: SOF: Mark get_ext* function ext_hdr arguments as const

This pointer can be mark as const to indicate that it is read
only pointer.

Signed-off-by: Karol Trzcinski <karolx.trzcinski@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Link: https://lore.kernel.org/r/20200415202816.934-3-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>