ASoC: Intel: Fix a self assignment in sst_mem_block_alloc_scratch()

Remove a self assignment in sst_mem_block_alloc_scratch(). When calculating
buffer sizes there is no need for statements without effect. Detected by
Coverity: CID 1195249.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: Mark Brown <broonie@linaro.org>