axge: Add support for AX88179A

The AX88179A has two firmware modes, one of which is backward
compatible with existing AX88178A/179 driver. The active firmware mode
can be controlled through a register.

Update axge(4) man page to mention 179A support and ensure that, when
bound to a AX88179A, the driver activates the compatible firmware mode.

Reviewed by: markj
Pull Request: https://github.com/freebsd/freebsd-src/pull/854
MFC after: 1 week