usb-ehci: drop assert()

Not sure what the purpose of the assert() was, in any case it is bogous.
We can arrive there if transfer descriptors passed to us from the guest
failed to pass sanity checks, i.e. it is guest-triggerable.  We deal
with that case by resetting the host controller.  Everything is ok, no
need to throw a core dump here.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>