ax88172a: fix ax88172a_unbind() failures

If ax88172a_unbind() fails, make sure that the return code is
less than zero so that cleanup is done properly and avoid UAF.

Fixes: a9a51bd7 ("ax88172a: fix information leak on short answers")
Signed-off-by: George Kennedy <george.kennedy@oracle.com>
Reported-by:  <syzbot+4cd84f527bf4a10fc9c1@syzkaller.appspotmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>