wfx: use container_of() to get vif

Currently, upon virtual interface creation, wfx_add_interface() stores
a reference to the corresponding struct ieee80211_vif in private data,
for later usage. This is not needed when using the container_of
construct. This construct already has all the info it needs to retrieve
the reference to the corresponding struct from the offset that is
already available, inherent in container_of(), between its type and
member inputs (struct ieee80211_vif and drv_priv, respectively).
Remove vif (which was previously storing the reference to the struct
ieee80211_vif) from the struct wfx_vif, define a function
wvif_to_vif(wvif) for container_of(), and replace all wvif->vif with
the newly defined container_of construct.

Signed-off-by: Jaehee Park <jhpark1013@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220506170046.GA1297231@jaehee-ThinkPad-X1-Extreme