cxgbe: Flush transmitted packets more regularly in netmap mode

Previously, when transmitting short runs of packets via cxgbe_nm_tx(),
we would wait until a large number of packets were buffered before
scheduling a task to clean transmit buffers.

Approved by: re (cperciva)
Obtained from: np

(cherry picked from commit 6af3d59962449c66688361598163dd174cc5fb2a)
(cherry picked from commit 59a9c20d7736d41be22791ec9a797cc535734ace)