ASoC: SOF: imx8: Fix an is IS_ERR() vs NULL check

The device_link_add() function only returns NULL on error, it doesn't
return error pointers.

Fixes: 202acc56 ("ASoC: SOF: imx: Add i.MX8 HW support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20190826131855.GA6840@mwanda


Signed-off-by: Mark Brown <broonie@kernel.org>