staging: usbip: vhci: fix oops on subsequent attach

vhci_rx/vhci_tx threads are created once but stopped each
time the vdev is shut down. On subsequent attach wake_up_process()
oopses trying to access the stopped threads.

This patch does as before the kthread conversion which is to
create the threads each time a device is attached and stop the
threads when the device is shut down.

Signed-off-by: Max Vozeler <max@hinterhof.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Takahiro Hirofuchi <hirofuchi@users.sourceforge.net>
Cc: Arjan Mels <arjan.mels@gmx.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>