net: Avoid gcc'ism in net_host_device_add

>> +    if (net_client_init(device, opts ? : "") < 0) {
>
> Is this a gcc extension?  Do we want to introduce this construct to the
> code base.

Valid remark, fix below.

Thanks,
Jan

-------->

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
