iflib tx watchdog doesn't appear that it can work anymore

[Ryan Stone]

https://svnweb.freebsd.org/base?view=revision&revision=356310

As of r356310, there does not appear to be a way that ift_qstatus can
be set to anything but IFLIB_QUEUE_IDLE.  As a result, I don't see
that there is any way for the watchdog to ever fire.

I think that iflib has missed setting ift_qstatus to
IFLIB_QUEUE_WORKING when packets are enqueued on the rx ring.