staging: r8188eu: use sa instead of Addr2

For management frames, Addr2 is the Source Address (SA).

Use sa from the mgmt structure and remove the GetAddr2Ptr call.
GetAddr2Ptr is a driver-specific function that we should eventually
remove.

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/20221015152440.232281-5-martin@kaiser.cx


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