staging: usbip: simplified errorhandling

In each errorcase spin_unlock_irq is called and -EINVAL is returned.
To simplify that I created a label called "err" doing that.
On Success count will be returned.

Signed-off-by: Kurt Kanzenbach <ly80toro@cip.cs.fau.de>
Signed-off-by: Stefan Reif <ke42caxa@cip.cs.fau.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>