i2c: slave: improve sanity check when registering

Add check for ERR_PTR and simplify code while here.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Alain Volmat <alain.volmat@st.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/i2c-core-slave.c

@@ -18,10 +18,8 @@ int i2c_slave_register(struct i2c_client *client, i2c_slave_cb_t slave_cb)
 {
 	int ret;
 
-	if (!client || !slave_cb) {
-		WARN(1, "insufficient data\n");
+	if (WARN(IS_ERR_OR_NULL(client) || !slave_cb, "insufficient data\n"))
 		return -EINVAL;
-	}
 
 	if (!(client->flags & I2C_CLIENT_SLAVE))
 		dev_warn(&client->dev, "%s: client slave flag not set. You might see address collisions\n",