[POWERPC] Fix builtin command line interaction with firmware

It seems that prom_init's early_cmdline_parse is broken on at least
Apple 970 xserves and IBM JS20 blades with SLOF.  The firmware of these
machines returns -1 and 1 respectively when getprop is called for the
bootargs property of /chosen, causing Linux to ignore its builtin
command line in favor of a null string.  This patch makes Linux use its
builtin command line if getprop returns an error or a null string.

Signed-off-by: Amos Waterland <apw@us.ibm.com>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>