riscv: remove the switch statement in do_trap_break()

To make the code more straightforward, replace the switch statement
with an if statement.

Suggested-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
[paul.walmsley@sifive.com: cleaned up patch description; updated to
 apply]
Link: https://lore.kernel.org/linux-riscv/20190927224711.GI4700@infradead.org/
Link: https://lore.kernel.org/linux-riscv/CABvJ_xiHJSB7P5QekuLRP=LBPzXXghAfuUpPUYb=a_HbnOQ6BA@mail.gmail.com/
Link: https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org/thread/VDCU2WOB6KQISREO4V5DTXEI2M7VOV55/


Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>