ASoC: amd: raven: acp3x-i2s: remove unused-but-set variable

Fix W=1 warning. The variable prtd is set but not used.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200702164433.162815-7-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>