This fixes a minor error in usb-skeleton's disconnect() routine: if the
interface's private data is NULL, the current code exits without releasing
the disconnect_sem semaphore.

I removed the test entirely because I can't think of any situation where
that private data actually would be NULL, other than a pretty badly
malfunctioning system.  Why test for something that should never happen?
And if it does happen, we shouldn't want the disconnect routine to fail
silently -- we should want to see a nice big segfault (when the NULL
pointer is dereferenced) so that we can find and fix the underlying error.

Is there any sort of convention (a la Documentation/CodingStyle) about
whether this approach should be used in general?  There are _lots_ of
places in the kernel where unnecessary checks for NULL pointers are made.