Speficies the mode in which the speed, duplex modes and flow control
of the interface is determined.
If set as 'auto', the hardware senses speed and negotiates with the port
on the other end of the link for data transfer operation as
'full-duplex' or 'half-duplex' and about flow contol.
If set as 'nonegotiation', the configured values for interface
speed, duplex mode and flow control will be effective.