cxgbe: Only run ktls_tick when NIC TLS is enabled.

Previously the body of ktls_tick was a nop when NIC TLS was disabled,
but the callout was still scheduled consuming power on otherwise-idle
systems with Chelsio T6 adapters. Now the callout only runs while NIC
TLS is enabled on at least one interface of an adapter.

Reported by: mav
Reviewed by: np, mav
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D32491