cxgbe(4): Always use the per-VI callout to read interface stats.

There is no change in the source of the stats (t4_get_port_stats or
t4_get_vi_stats) but the per-port callout is gone.

Sponsored by: Chelsio Communications
Reviewed by: jhb@
Differential Revision: https://reviews.freebsd.org/D29527

(cherry picked from commit 516fe911a6b7a2d2f245ebbc7837e5dc2e353703)