brcm63xx-arm: fix media type setting with autoneg disabled in /lib/network/port.sh (IOP-6415)

Commit 7d1285b6 introduced an issue with setting media type for a particular port when
autonegotiation is disabled. In this case $subport ends up being undefined, resulting in an error in ethctl, which
leads to the appropriate media type not being set.

This change is designed to fix the above. Additionally cleaned up the conditionals a bit, so that media type is only
determined once and then reused regardless of whether "oncrossbar" is set or not.

Related to IOP-6415
Related to IOP-5001