ethtool: use ethnl_parse_header_dev_put()

It seems I missed that most ethnl_parse_header_dev_get() callers
declare an on-stack struct ethnl_req_info, and that they simply call
dev_put(req_info.dev) when about to return.

Add ethnl_parse_header_dev_put() helper to properly untrack
reference taken by ethnl_parse_header_dev_get().

Fixes: e4b89540 ("netlink: add net device refcount tracker to struct ethnl_req_info")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>