Staging: speakup: Fix return value in synth.c

The function return type is a pointer. Hence return NULL instead
of 0.
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/speakup/synth.c

@@ -157,7 +157,7 @@ const char *spk_synth_immediate(struct spk_synth *synth, const char *buff)
 			return buff;
 		buff++;
 	}
-	return 0;
+	return NULL;
 }
 EXPORT_SYMBOL_GPL(spk_synth_immediate);