LoongArch: Fix unsigned comparison with less than zero

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>

arch/loongarch/kernel/unwind_prologue.c

@@ -43,7 +43,8 @@ static bool unwind_by_prologue(struct unwind_state *state)
 {
 	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)