cxgbe(4): Fix an incorrect assert.

CTRL and OFLD tx queues do not have automatic tx credit flush enabled so
it is okay for the cidx not to be the same as the pidx when the queue is
destroyed.

Reported by: Jithesh Arakkan @ Chelsio
Sponsored by: Chelsio Communications

(cherry picked from commit 5ef87bf8b687575bee010967e23cd2c552b43ad9)