ASoC: dmaengine: fix deferred probe detection

Check the return value of dma_request_slave_channel_reason() to see if
deferred probe happens, not the variable the return value will be
assigned to later.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 5eda87b8 ("ASoC: dmaengine: support deferred probe for DMA channels")
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>