ASoC: rt5514-spi: Fix potential NULL pointer dereference

There is a potential NULL pointer dereference in case devm_kzalloc()
fails and returns NULL.

Fix this by adding a NULL check on rt5514_dsp.

This issue was detected with the help of Coccinelle.

Fixes: 6eebf35b

 ("ASoC: rt5514: add rt5514 SPI driver")
Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Mark Brown <broonie@kernel.org>