staging: r8188eu: fix rtw_xmitframe_enqueue error handling

In function rtw_xmitframe_enqueue, we can simply relay the error code from
rtw_xmit_classifier.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20230123205342.229589-20-martin@kaiser.cx


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