cxgbe: Fix some merge-o's for the per-rxq iSCSI counters.

I botched a few of the changes when rebasing the changes in
4b6ed0758dc6fad17081d7bd791cb0edbddbddb8 across the changes in
43bbae19483fbde0a91e61acad8a6e71e334c8b8.

• Move the counter allocations into alloc_ofld_rxq().

• Free the counters freeing an ofld rxq.

Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D30267