RISC-V: Fix missing break statement in disassembler

RISC-V: Fix missing break statement in disassembler

This fixes an issue when disassembling rv128 c.sqsp,
where the code erroneously fell through to c.swsp.

Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Clark <mjc@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>