staging: usbip: Fix deadlock

When detaching a port from the client side (usbip --detach 0),
the event thread, on the server side, is going to deadlock.
The "eh" server thread is getting USBIP_EH_RESET event and calls:
  -> stub_device_reset() -> usb_reset_device()
the USB framework is then calling back _in the same "eh" thread_ :
  -> stub_disconnect() -> usbip_stop_eh() -> wait_for_completion()
the "eh" thread is being asleep forever, waiting for its own completion.
This patch checks if "eh" is the current thread, in usbip_stop_eh().

Signed-off-by: Eric Lescouet <eric@lescouet.org>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>