MIPS: eBPF: Make ebpf_to_mips_reg() static

Fix sparse warning:

arch/mips/net/ebpf_jit.c:196:5: warning:
 symbol 'ebpf_to_mips_reg' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>