MIPS: Fix a copy & paste error in unistd.h

Commit 5df4c8dbbc (MIPS: Wire up bpf syscall.) break the N32 build
because of a copy & paste error.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Cc: John Crispin <john@phrozen.org>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/8390/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>