staging: r8188eu: replace one GetAddr3Ptr call

Define a struct ieee80211_mgmt for the message that we process in
OnDeAuth. Use this struct to read the bssid.

This patch removes one GetAddr3Ptr call, getting us a tiny step closer to
removing GetAddr3Ptr.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Acked-by: Pavel Skripkin <paskripkin@gmail.com>
Link: https://lore.kernel.org/r/20221015152440.232281-2-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>