Documentation: networking: fix ethtool-netlink table formats

Fix table formatting to eliminate warnings.

Documentation/networking/ethtool-netlink.rst:509: WARNING: Malformed table.
Documentation/networking/ethtool-netlink.rst:522: WARNING: Malformed table.
Documentation/networking/ethtool-netlink.rst:543: WARNING: Malformed table.
Documentation/networking/ethtool-netlink.rst:555: WARNING: Malformed table.
Documentation/networking/ethtool-netlink.rst:591: WARNING: Malformed table.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>