staging: r8188eu: simplify err handling for unknown station

If we receive a disassoc message from an unknown station, we can drop this
message immediately. Reorder the code to make this clearer.

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/20221129205152.128172-6-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>