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>