ASoC: sprd: Fix the smatch warning

Remove the unnecessary validation of the 'cstream' variable to fix
below smatch warning:

sprd_platform_compr_drain_notify() warn: variable dereferenced
before check 'cstream' (see line 105)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sprd/sprd-pcm-compress.c

@@ -107,8 +107,7 @@ static void sprd_platform_compr_drain_notify(void *arg)
 
 	memset(stream->info_area, 0, sizeof(struct sprd_compr_playinfo));
 
-	if (cstream)
-		snd_compr_drain_notify(cstream);
+	snd_compr_drain_notify(cstream);
 }
 
 static void sprd_platform_compr_dma_complete(void *data)