staging: r8188eu: replace one GetAddr3Ptr call

Define a struct ieee80211_mgmt in the OnBeacon function. Use it to replace
one GetAddr3Ptr call. This is a tiny step towards making GetAddr3Ptr
obsolete.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20221126160129.178697-2-martin@kaiser.cx


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