staging: r8188eu: simplify error handling for missing station

Simplify the code to handle the case where we're associated to a station
that is not in our list of known stations.

We can simply exit in this case. This patch reverts the if-condition and
saves one level of indentation.

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/20221126160129.178697-10-martin@kaiser.cx


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