Staging: speakup/speakup_dtlk: Return NULL instead of 0

The function returns a pointer. Hence return NULL instead of 0.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/speakup/speakup_dtlk.c

@@ -254,7 +254,7 @@ static const char *synth_immediate(struct spk_synth *synth, const char *buf)
 		spk_out(ch);
 		buf++;
 	}
-	return 0;
+	return NULL;
 }
 
 static void synth_flush(struct spk_synth *synth)