cfg80211: simplify netlink socket owner interface deletion

cfg80211: simplify netlink socket owner interface deletion

There's no need to allocate a portid structure and then, for
each of those, walk the interfaces - we can just add a flag
to each interface and walk those directly. Due to padding in
the struct, we can even do it without any memory cost, and
it even simplifies the code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>