USB: fix memleak in usbfs

This patch fixes a memory leak in devio.c::processcompl

If writing to user space fails the packet must be discarded, as it
already has been removed from the queue of completed packets.

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>