ethtool: potential NULL dereference in strset_prepare_data()

Smatch complains that the NULL checking isn't done consistently:

    net/ethtool/strset.c:253 strset_prepare_data()
    error: we previously assumed 'dev' could be null (see line 233)

It looks like there is a missing return on this path.

Fixes: 71921690 ("ethtool: provide string sets with STRSET_GET request")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>