xtensa: iss/network: drop 'devices' list

There are two per-device lists in the ISS network driver: command line
parameters list and iss_net_private object list. The latter is only used
for duplicate checking in the function iss_net_setup where the former
should have been used.
Drop iss_net_private object list and associated code and use command
line parameters list in the iss_net_setup instead.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>