iflib: Clear simple-TX quiescence before publishing admission

[kbowling]

Summary

Initialization publishes RUNNING and enables interrupts before clearing
the simple-TX producer barrier. A transmitter admitted in that window
still sees IFLIB_TXQ_QUIESCING and drops its packet with ENETDOWN. A TX
completion task can likewise return without draining deferred packets.

Clear the barrier on every TX queue after driver initialization and RX
buffer setup succeed, before publishing admission. Keep failed init
paths closed and retain the existing stop, interrupt and timer ordering.
This changes only initialization; no work is added to the packet path.