csky/syscall_trace: Fixup return processing flow

The function tracehook_report_syscall_entry's return value is
__must_check attribute. We should add return processing flow in
ptrace.c and set the syscall number to -1 when failed just like
riscv's.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>