Convert bge(4) to new media and miibus KPI.

Tested: Broadcom NetXtreme Gigabit Ethernet Controller, ASIC rev. 0x001002
Tested: Broadcom Gigabit Ethernet Controller, ASIC rev. 0x007102
Tested: Broadcom NetXtreme Gigabit Ethernet Controller, ASIC rev. 0x000105
Sponsored by: Netflix
Sponsored by: Nginx, Inc.