cxgbe: Tidy TOE tunables under hw.cxgbe

[jhb]

Summary

• Only two of these tunables are used for RATELIMIT without TCP_OFFLOAD.

• Mark t4_tmr_idx_ofld and t4_pktc_idx_ofld static.

• Move hw.cxgbe.cop_managed_offloading under hw.cxgbe.toe since it is specific to TOE.

[jhb]

Note to self: need to update cxgbetool.8 manpage which references the hw.cxgbe.cop_managed_offloading tunable.