usb: typec: anx7411: Fix return value check in anx7411_register_i2c_dummy_clients()

If i2c_new_dummy_device() fails, it never return NULL pointer, replace
NULL test with IS_ERR() to fix it.

Fixes: fe6d8a9c

 ("usb: typec: anx7411: Add Analogix PD ANX7411 support")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220721071201.269344-2-yangyingliang@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>