iw: fix "iw reg get" double output

iw: fix "iw reg get" double output

Axel reports that running "iw reg get" results in the (global)
output being printed twice. The reason for this is that we try
to use the dump facility, and if that succeeds we also do the
get command.

To prevent this, allow handlers to return HANDLER_RET_DONE, in
which case the command will be treated as successful but will
not actually execute another netlink command.

Reported-by: Axel Köllhofer <AxelKoellhofer@web.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>