qtnfmac: do not initialize per-MAC data multiple times

Several members of pwr-MAC structure are re-initialized several times
together with per-VIF initialization. Fix that and simplify the code.

Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>