ethtool: fix string set id check

Syzbot reported a shift of a u32 by more than 31 in strset_parse_request()
which is undefined behavior. This is caused by range check of string set id
using variable ret (which is always 0 at this point) instead of id (string
set id from request).

Fixes: 71921690

 ("ethtool: provide string sets with STRSET_GET request")
Reported-by:  <syzbot+96523fb438937cd01220@syzkaller.appspotmail.com>
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Link: https://lore.kernel.org/r/b54ed5c5fd972a59afea3e1badfb36d86df68799.1607952208.git.mkubecek@suse.cz


Signed-off-by: Jakub Kicinski <kuba@kernel.org>