openvswitch: Store flow key len if ARP opcode is not request or reply.

We currently only extract the ARP payload if the opcode indicates
that it is a request or reply.  However, we also only set the
key length in these situations even though it should still be
possible to match on the opcode.  There's no real reason to
restrict the ARP opcode since all have the same format so this
simply removes the check.

Signed-off-by: Mehak Mahajan <mmahajan@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>