Clear RX completion queue state veriables in iflib_stop()

iflib_stop() was not resetting the rxq completion queue state variables.
This meant that for any driver that has receive completion queues, after a
reinit, iflib would start asking what's available on the rx side starting at
whatever the completion queue index was prior to the stop, instead of at 0.

Submitted by: pkelsey
Reported by: pkelsey
MFC after: 3 days
Sponsored by: Limelight Networks