feat(mt8188): update return value in mtk_emi_mpu_sip_handler

Remove the use of SMC_RET2 in the mtk_emi_mpu_sip_handler function. The
current smc driver in the atf driver has switched to using SMC_RET4 for
smc call clients. This change aligns the return value handling with the
updated driver behavior that ensures consistency and avoids potential
issues with the old return value.

Change-Id: I87f25b438d2119837c45bed80a8224fcfd141fb6
Signed-off-by: Dawei Chien <dawei.chien@mediatek.com>
Signed-off-by: Jason Chen <Jason-ch.Chen@mediatek.com>

plat/mediatek/drivers/emi_mpu/emi_mpu_common.c

@@ -118,12 +118,7 @@ u_register_t mtk_emi_mpu_sip_handler(u_register_t x1, u_register_t x2,
 				     u_register_t x3, u_register_t x4,
 				     void *handle, struct smccc_res *smccc_ret)
 {
-	int ret;
-
-	ret = emi_mpu_optee_handler(x1, x2, x3);
-	SMC_RET2(handle, ret, 0U);
-
-	return 0;
+	return (u_register_t) emi_mpu_optee_handler(x1, x2, x3);
 }
 DECLARE_SMC_HANDLER(MTK_SIP_TEE_MPU_PERM_SET, mtk_emi_mpu_sip_handler);