[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>

arch/powerpc/kernel/prom_init.c

@@ -567,7 +567,7 @@ static void __init early_cmdline_parse(void)
 	if ((long)_prom->chosen > 0)
 		l = prom_getprop(_prom->chosen, "bootargs", p, COMMAND_LINE_SIZE-1);
 #ifdef CONFIG_CMDLINE
-	if (l == 0) /* dbl check */
+	if (l <= 0 || p[0] == '\0') /* dbl check */
 		strlcpy(RELOC(prom_cmd_line),
 			RELOC(CONFIG_CMDLINE), sizeof(prom_cmd_line));
 #endif /* CONFIG_CMDLINE */