cxgbe(4): Revert r367917.

r367917 fixed the backpressure on the netmap rxq being stopped but that
doesn't help if some other netmap rxq is starved (because it is stopping
too although the driver doesn't know this yet) and blocks the pipeline.
An alternate fix that works in all cases will be checked in instead.

Sponsored by: Chelsio Communications

(cherry picked from commit f42f3b2955e5afa770275fd76b4f754fbd67bbeb)