rapidio: add check for rio_add_net() in rio_scan_alloc_net()

In the Linux kernel, the following vulnerability has been resolved:

rapidio: add check for rio_add_net() in rio_scan_alloc_net()

The return value of rio_add_net() should be checked. If it fails,

put_device() should be called to free the memory and give up the reference

initialized in rio_add_net().