csky: Fixup wrong pt_regs size

The bug is from commit 2054f4af

 ("csky: bugfix gdb coredump error.")

We change the ELF_NGREG to ELF_NGREG - 2 to fit gdb&gcc define, but forgot
modify ptrace regset.
Now coredump use ELF_NRGEG to parse GPRs and ptrace use pt_regs_regset, so
there are two different reg_sets for userspace.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>