ASoC: xtfpga-i2s: Add missing __rcu annotation

tx_substeam is accessed using the RCU API and hence should have the __rcu
annotation. Fixes the following sparse warnings:

	sound/soc/xtensa/xtfpga-i2s.c:165:24: error: incompatible types in
		comparison expression (different address spaces)
	sound/soc/xtensa/xtfpga-i2s.c:165:24: error: 'struct snd_pcm_substream
		[noderef] <asn:4>*' versus
	sound/soc/xtensa/xtfpga-i2s.c:165:24: error: 'struct snd_pcm_substream *'
	sound/soc/xtensa/xtfpga-i2s.c:255:24: error: incompatible types in
		comparison expression (different address spaces)
	sound/soc/xtensa/xtfpga-i2s.c:255:24: error: 'struct snd_pcm_substream
		[noderef] <asn:4>*' versus
	sound/soc/xtensa/xtfpga-i2s.c:255:24: error: 'struct snd_pcm_substream *'
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/xtensa/xtfpga-i2s.c

@@ -75,7 +75,7 @@ struct xtfpga_i2s {
 	 * stream in the pcm_close callback it synchronizes with the interrupt
 	 * handler by means of synchronize_rcu call.
 	 */
-	struct snd_pcm_substream *tx_substream;
+	struct snd_pcm_substream __rcu *tx_substream;
 	unsigned (*tx_fn)(struct xtfpga_i2s *i2s,
 			  struct snd_pcm_runtime *runtime,
 			  unsigned tx_ptr);