MIPS: loongson2ef: Add missing break in cs5536_isa

Fixes build error:

arch/mips/loongson2ef/common/cs5536/cs5536_isa.c:217:2: error:
unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        default:
        ^
arch/mips/loongson2ef/common/cs5536/cs5536_isa.c:217:2:
note: insert 'break;' to avoid fall-through
        default:
        ^
        break;

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>