uether: Handle SIOCGIFXMEDIA

[emaste]

Summary

Commit eb7e25b22f1 ("ifmedia changes:") added SIOCGIFXMEDIA for extended
Ethernet media subtypes, but uether_ioctl() was not updated to pass it
to ifmedia_ioctl().  I believe no existing USB Ethernet devices use
extended subtypes, so this went unnoticed.

Without SIOCGIFXMEDIA ifconfig falls back to SIOCGIFMEDIA, which maps an
extended subtype to IFM_OTHER, and ifconfig reports "Ethernet Other."