staging: r8188eu: replace GetAddr2Ptr calls

Get the source address of a beacon frame from our struct ieee80211_mgmt
instead of using the driver's internal GetAddr2Ptr helper.

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/20221126160129.178697-4-martin@kaiser.cx


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