MFC 343456: Fix a few more places to handle ofld tx queues for RATELIMIT.
- Drain offload transmit queues when RATELIMIT is enabled but TCP_OFFLOAD is not.
- Expose the per-VI nofldtxq and first_ofld_txq sysctls when RATELIMIT is enabled but TCP_OFFLOAD is not.
- Clear offload transmit queue stats as part of a 'cxgbetool clearstats' request when RATELIMIT is enabled but TCP_OFFLOAD is not.