rtwn: add APIs for setting transmit power

[adrian]

Summary

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.

[bz]

I think at least r92eu_attach is lacking the default initialization?

[adrian]

from bz - add missing init

[adrian]

In D47979#1097408, @bz wrote:

I think at least r92eu_attach is lacking the default initialization?

fixed, good catch!

(I added the initialiser in a later commit in the stack, which is why I didn't catch it here..)

[adrian]

update from another rebase attempt

[adrian]

update from rebase