ASoC: Intel: atom: Fix uninitialized variable compiler warning

GCC 10 gives a "variable might be used uninitialized" warning for the
block variable in sst_prepare_and_post_msg().

This is a false-positive warning, but lets fix it anyways.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200402185359.3424-3-hdegoede@redhat.com
Signed-off-by: Mark Brown <broonie@kernel.org>