b43: implement PPR (Power Per Rate) management/API

Broadcom hardware supports auto-adjustment of TX power depending on the
currently used rate. So far all calculations were handled without any
helpers (API) using big arrays and magic offsets.
It seems Broadcom recently decided to clean this up by developing PPR.
Their wlc_ppr.h can be found in open parts of the SDK.
As we plan to implement support for rate-based TX power it makes sense
to also implement our version of PPR as well.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>