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>