rtwn: add tx power training for RTL8812/RTL8821

This apparently kicks off TX power level self-calibration, which
can't hurt.

Locally tested:

• RTL8812AU, STA
• RTL8821AU, STA

Obtained from: Linux rtw88

Differential Revision: https://reviews.freebsd.org/D47978

Reviewed by: bz, imp