ASoC: Intel: Atom: sst: remove useless NULL assignment

Fix cppcheck warnings:

sound/soc/intel/atom/sst/sst.c:373:2: warning: Assignment of function
parameter has no effect outside the function. Did you forget
dereferencing it? [uselessAssignmentPtrArg]
 ctx = NULL;
 ^
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200813200147.61990-5-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/intel/atom/sst/sst.c

@@ -370,7 +370,6 @@ void sst_context_cleanup(struct intel_sst_drv *ctx)
 	kfree(ctx->fw_in_mem);
 	ctx->fw_in_mem = NULL;
 	sst_memcpy_free_resources(ctx);
-	ctx = NULL;
 }
 EXPORT_SYMBOL_GPL(sst_context_cleanup);