Only check GiGE related flags in the generic code when MIIF_IS_1000X is set

Only check GiGE related flags in the generic code when MIIF_IS_1000X is set
in mii_softc->mii_flags.  This really only effects the generic MII code, as
the flags for GigE may not be compatible with all devices (such as xl(4)).

Change the mii_add_media() API to pass mii_softc instead of mii_data, and
derive the instance id from the softc instead of passing it.

Reported-by: Sepherosa Ziehau <sepherosa@gmail.com>