Fix a few more places to handle ofld tx queues for RATELIMIT.

[jhb]

Summary

• 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.

Test Plan

• compile tested