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

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

Fixes:	516fe911a6 cxgbe(4): Always use the per-VI callout to read interface stats.
Sponsored by:	Chelsio Communications

(cherry picked from commit 8b144c015c9cce0bc99a7fbdc43f22f51a946d2c)

sys/dev/cxgbe/t4_main.c

@@ -2733,6 +2733,7 @@ cxgbe_vi_detach(struct vi_info *vi)
 #endif
 	cxgbe_uninit_synchronized(vi);
 	callout_drain(&vi->tick);
+	mtx_destroy(&vi->tick_mtx);
 	sysctl_ctx_free(&vi->ctx);
 	vi_full_uninit(vi);