brcmfmac: Fix set and get tx-power functions.

Implementation of tx-power (get and set) related functions are
still assuming mW interface. This is wrong as functions use dbm
(or mbm) nowadays. As a result a tx power configuration could
result in wrong power configuration.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>