cxgbe: Allow parse_pkt to internally queue a packet.

If parse_pkt returns EINPROGRESS, return from cxgbe_transmit
without queueing the packet in a txq. Use this to move the call
to ethofld_transmit for packet pacing into parse_pkt.

Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D38577