if_muge: add LAN7850 support

Differences between LAN7800 and LAN7850 from the driver's perspective:

• The LAN7800 muxes EEPROM signals with LEDs, so LED mode needs to be disabled when reading/writing EEPROM. The EEPROM is not muxed on the LAN7850.

• The Linux driver enables automatic duplex and speed detection when there is no EEPROM, for the LAN7800 only. With this FreeBSD driver LAN7850-based adapters without a configuration EEPROM fail to link (with or without the automatic duplex and speed detection code), so I have just followed the example of the Linux driver for now.

Sponsored by: The FreeBSD Foundation
Sponsored by: Microchip (hardware)