When link auto-negotiation is enabled, by default the network device
advertises all the supported speed and duplex modes in order to
negotiate the fastest link speed with the remote endpoint.
It is possible anyway to configure the device to just advertise and
accept a subset of supported modes.
This could be useful to properly enforce gigabit speeds on Ethernet:
as stated in IEEE 802.3 specification, auto-negotiation is mandatory
for 1000Base-T and 10GBase-T standards.
Allow specific values to 802-3-ethernet.speed and 802-3-ethernet.duplex
properties also when 802-3-ethernet.auto-negotiate=yes: this will
result in link auto-negotiation advertising the specified speed/duplex
mode as the only one available.