[POWERPC] Fix panic in RTAS code

Some older pSeries machines were panicking in pSeries_log_error
because it was getting called before it was ready.  This is a result
of commit "[POWERPC] pseries: Fix jumbled no_logging flag."
(79c0108d).

This fixes it by explicitly enabling RTAS error logging when it has
been initialized, and also makes the code clearer by renaming the
"no_more_logging" variable to "logging_enabled".

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>