Improve netmap TX handling when TX IRQs are not used/supported

Use the timer to poll for TX completions when there are
outstanding TX slots. Track when the last driver timer was called
to prevent overcalling it. Also clean up some kring vs NIC ring
usage.

Reviewed by: marius, Johannes Lundberg <johalun0@gmail.com>
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D16300