emx: Add EMX_TSO_DEBUG

emx: Add EMX_TSO_DEBUG

It is used to measure TSO context reuse rate.  Setting up TX context
interferes TX data fetching pipelining, thus reduces performance, so
TX context setting up should be avoided as much as possible.

According to limited measurement:
- 1 one direction TX stream, reuse rate is ~40%
- 16 one direction TX streams, reuse rate is ~20%
- 16 bi-direction streams (total 32), reuse rate is ~10%