Improve TX performance of i8254x (PA-GE / PA-2FE-TX / C7200-IO-2FE)

Send up to 16 packets in task sleep interval. Based on approach in
dev_mueslix.c
Merge pull request #45 from candlerb/candlerb/txperformance

Improve TX performance of i8254x (PA-GE) by sending up to 16 packets at a time.

Link: http://forum.gns3.net/topic10476.html