e6000sw: stop / drain the taskqueue (and tick) during detach

Although the tick isn't running every hz right now, when it /is/
running at hz, the shutdown path will race with an existing running
tick routine, causing unpredictable panics.

• Introduce a shutdown flag which will abort doing the tick work if set
• set the shutdown flag and start cancel/draining the taskqueue during detach.

Differential Revision: https://reviews.freebsd.org/D50030