ASoC: dmaengine: do not use a NULL prepare_slave_config() callback

ASoC: dmaengine: do not use a NULL prepare_slave_config() callback

[ Upstream commit 9a1e13440a4f2e7566fd4c5eae6a53e6400e08a4 ]

Even if struct snd_dmaengine_pcm_config is used, prepare_slave_config()
callback might not be set. Check if this callback is set before using it.

Fixes: fa654e085300 ("ASoC: dmaengine-pcm: Provide default config")
Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Link: https://lore.kernel.org/r/20220307122202.2251639-2-codrin.ciubotariu@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>