staging: r8188eu: replace a GetAddr1Ptr call

Define a struct ieee80211_mgmt and use it to read the destination address.

This replaces one call to the driver-specific GetAddr1Ptr function, which
should eventually be removed.

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/20221030173326.1588647-2-martin@kaiser.cx


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