Replace the static "qdat" structure with a per-instance softc field

Replace the static "qdat" structure with a per-instance softc field
in all USB ethernet drivers. The qdat structure contains a pointer
to the interface's struct ifnet and is used to process incoming
packets, so simultaneous use of two similar devices caused crashes
and confusion.

The if_udav driver appeared in the tree since Daan's PR, so I made
similar changes to that driver too.

PR:		kern/59290
Submitted by:	Daan Vreeken <Danovitsch@Vitsch.net>