staging: r8188eu: use ieee80211 helper for source address

Use the ieee80211_get_SA helper to get a pointer to the source
address of the incoming data frame.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220403165438.357728-2-martin@kaiser.cx


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