uml: fix nonremovability of watchdog

The UML watchdog driver was using the wrong config variable to control whether
it can be unloaded once active.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>