wcn36xx: Fetch private sta data from sta entry instead of from vif

For consistency with other code.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
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>
wcn36xx: Remove sta pointer in private vif struct

This does not work with multiple sta's in a vif.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>