[MIPS] save_context_stack fix

CONFIG_KALLSYMS=n case is obviously wrong, though it is harmless since
CONFIG_KALLSYMS is always enabled with CONFIG_STACKTRACE for now.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>