ehci: Assert state machine is sane w.r.t. EHCIQueue

Coverity worries the EHCIQueue pointer could be null when we pass it
to functions that reference it.  The state machine ensures it can't be
null then.  Assert that, to hush the checker.

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