wcn36xx: Add helper macros to cast vif to private vif and vice versa

Makes the code a little easier to read.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
wcn36xx: Use consistent name for private vif

Some code used priv_vif and some used vif_priv. Convert all to vif_priv
for consistency.

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