Fix a bug that media setting for fiber didn't work correctly. Not tested.
The BUGS section in gem(4) manpage says:

> BUGS
(snip)
>      On the SX fibre variants of the hardware, the link will stay down if
>      there is a duplex mismatch.  Also, packet transmission may fail when in
>      half-duplex mode.

Is the bug related to this fix?

(msaitoh)