cxgbe: Tidy TOE tunables under hw.cxgbe

• 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.

Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D47765