powerpc: Fix crash in early boot on some powermacs

Some powermac machines were crashing in the quiesce firmware call
in prom_init.c because we have just closed the OF stdin device;
notably my 1999 G3 powerbook does this.  To avoid this, don't
close the OF stdin device on powermacs.

Signed-off-by: Paul Mackerras <paulus@samba.org>