rtwn: refactor out the TX power register power dump, condense output
- Refactor out the TX power register register dump - it's done in a couple places and it makes sense to refactor it.
- Condense the output into a few lines per transmit chain. It's very long with the 8 and 16 MCS rates, and it made it difficult to eyeball what's going on when tweaking TX power.
Differential Revision: https://reviews.freebsd.org/D47986
Reviewed by: bz, imp