rtwn: add APIs for setting transmit power

The RTL8188/RTL8192/RTL8821/RTL8812 NICs all seem happy to have
their transmit power changed at runtime - and it does seem to do
what's expected - the transmit power level does change.

So, add the API call here, even though it's all currently no-ops.
A follow-up commit will land changes for the chipsets to both
limit transmit power to the configured / regulatory limit AND
allow reconfiguration at runtime.

Differential Revision: https://reviews.freebsd.org/D47979
Reviewed by: bz, imp