Some Gigabit Ethernet compatible SFP transeivers, such as
SFP-FDSGMII-M and SFP-FDSGMII-LR, have a built-in SGMII PHY.
Those that do require special autonegotiation processing,
without which autonegotiation will fail and the port will be
unable to pass traffic.
If the SFP has a built-in SGMII PHY, the user should set this
to yes(3).