MIPS,bpf: Cache value of BPF_OP(insn->code) in eBPF JIT.

The code looks a little cleaner if we replace BPF_OP(insn->code) with
the local variable bpf_op.  Caching the value this way also saves 300
bytes (about 1%) in the code size of the JIT.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>