The earlier commit 8d927391
(Consolidate early console and PPCDBG code) broke iSeries because
it caused unregister_console(&udbg_console) to be called
unconditionally.  iSeries never registers the udbg_console.

This just reverts part of the change.
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>