ASoC: rt5677: Add missing null check for failed allocation of rt5677_dsp

The allocation of rt5677_dsp can potentially fail and return null, so add
a null check and return -ENOMEM on a memory allocation failure.

Addresses-Coverity: ("Dereference null return")
Fixes: a0e0d135

 ("ASoC: rt5677: Add a PCM device for streaming hotword via SPI")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20191024124610.18182-1-colin.king@canonical.com
Signed-off-by: Mark Brown <broonie@kernel.org>