csky: Fixup duplicated restore sp in RESTORE_REGS_FTRACE

There is no user return for RESTORE_REGS_FTRACE, so it's no need to
save sp into ss0 as RESTORE_REGS_ALL.

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