wcn36xx: Add helper macros to cast sta to priv

While poking at this I also change two related things. I rename one
variable to make the names consistent. I also move one assignment of
priv_sta to the declaration to save a few lines.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>