net: ethtool: add support for 2500BaseT and 5000BaseT link modes

This patch introduce support for 2500BaseT and 5000BaseT link modes.
These modes are included in the new IEEE 802.3bz standard.

Signed-off-by: Pavel Belous <pavel.s.belous@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>