csky: Fixup kprobes handler couldn't change pc

The "Changing Execution Path" section in the Documentation/kprobes.txt
said:

Since kprobes can probe into a running kernel code, it can change the
register set, including instruction pointer.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Arnd Bergmann <arnd@arndb.de>