nl80211: move unsplit command advertising to a separate function

When we split the wiphy dump because it got too large, I added a
comment and asked that all new command advertising be done only
for userspace clients capable of receiving split data, in order
to not break older ones (which can't use the new commands anyway)

This mostly worked, and we haven't added many new commands, but
I occasionally get patches that modify the wrong place.

Make this easier to detect and understand by splitting out the
old commands to a separate function that makes it more clear it
should never be modified again.

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