iw: handle positive error codes gracefully

iw: handle positive error codes gracefully

netlink(7) requires error codes to be negative, but since when does a
man page stop anyone? At a minimum, we shouldn't allow a non-conforming
vendor command to put us into an infinite loop in the below snippets
from __handle_cmd():

	err = 1;

	nl_cb_err(cb, NL_CB_CUSTOM, error_handler, &err);
...
	while (err > 0)
		nl_recvmsgs(state->nl_sock, cb);

Signed-off-by: Brian Norris <briannorris@chromium.org>
Link: https://lore.kernel.org/r/20201103235631.2936594-1-briannorris@chromium.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>