Blackfin: simplify SYSCFG code a bit and ignore attempts to change it

We don't want to let user space modify the SYSCFG register arbitrarily as
the settings are system wide (SNEN/CNEN) and can cause misbehavior.  The
only other bit here (SSSTEP) has proper controls via PTRACE_SINGLESTEP.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>