ASoC: qdsp6: q6asm-dai: Fix a NULL vs IS_ERR() bug

The q6asm_audio_client_alloc() doesn't return NULL, it returns error
pointers.

Fixes: 22930c79 ("ASoC: qdsp6: q6asm-dai: Add support to compress offload")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>