staging: usbip: vhci: refuse to enqueue for dead connections

There can be requests to enqueue URBs while we are shutting
down a connection.

Signed-off-by: Max Vozeler <max@vozeler.com>
Tested-by: Mark Wehby <MWehby@luxotticaRetail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>