Eliminate the following coccicheck warning:
./arch/loongarch/kernel/unwind_prologue.c:84:5-13: WARNING: Unsigned
expression compared with zero: frame_ra < 0
Signed-off-by: KaiLong Wang <wangkailong@jari.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
 {
        struct stack_info *info = &state->stack_info;
        union loongarch_instruction *ip, *ip_end;
-       unsigned long frame_size = 0, frame_ra = -1;
+       long frame_ra = -1;
+       unsigned long frame_size = 0;
        unsigned long size, offset, pc = state->pc;
 
        if (state->sp >= info->end || state->sp < info->begin)