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 <[e-mail]>
Reviewed-by: Peng Fan <[e-mail]>
Reviewed-by: Abel Vesa <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Abel Vesa <[e-mail]>

drivers/clk/imx/clk-imx8-acm.c

···
 							 DL_FLAG_STATELESS |
 							 DL_FLAG_PM_RUNTIME |
 							 DL_FLAG_RPM_ACTIVE);
-		if (IS_ERR(dev_pm->pd_dev_link[i])) {
+		if (!dev_pm->pd_dev_link[i]) {
 			dev_pm_domain_detach(dev_pm->pd_dev[i], false);
-			ret = PTR_ERR(dev_pm->pd_dev_link[i]);
+			ret = -EINVAL;
 			goto detach_pm;
 		}
 	}