ethtool: rename ethnl_parse_header() to ethnl_parse_header_dev_get()

Andrew Lunn pointed out that even if it's documented that
ethnl_parse_header() takes reference to network device if it fills it
into the target structure, its name doesn't make it apparent so that
corresponding dev_put() looks like mismatched.

Rename the function ethnl_parse_header_dev_get() to indicate that it
takes a reference.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>