csky: syscache: Fixup duplicate cache flush

The current csky logic of sys_cacheflush is wrong, it'll cause
icache flush call dcache flush again. Now fixup it with a
conditional "break & fallthrough".

Fixes: 997153b9 ("csky: Add flush_icache_mm to defer flush icache all")
Fixes: 0679d29d ("csky: fix syscache.c fallthrough warning")
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Co-Developed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Arnd Bergmann <arnd@arndb.de>