cxgbe(4): Destroy the tick mutex during VI detach.

This avoids a mutex reinitialization when the VI is detached and
reattached.

Fixes: 516fe911a6b7 cxgbe(4): Always use the per-VI callout to read interface stats.
Sponsored by: Chelsio Communications

(cherry picked from commit 8b144c015c9cce0bc99a7fbdc43f22f51a946d2c)