clk: imx: imx8-acm: Fix return value check in clk_imx_acm_attach_pm_domains()

clk: imx: imx8-acm: Fix return value check in clk_imx_acm_attach_pm_domains()

If device_link_add() fails, it returns NULL pointer not ERR_PTR(),
replace IS_ERR() with NULL pointer check, and return -EINVAL.

Fixes: d3a0946d7ac9 ("clk: imx: imx8: add audio clock mux driver")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Abel Vesa <abel.vesa@linaro.org>
Link: https://lore.kernel.org/r/20241026112452.1523-1-yangyingliang@huaweicloud.com
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>