ASoC: qdsp6: q6asm-dai: checking NULL vs IS_ERR()

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

Fixes: 2a9e92d3 ("ASoC: qdsp6: q6asm: Add q6asm dai driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>