staging: r8188eu: simplify dequeue_one_xmitframe

Revert the if condition and exit immediately if the list of xmitframes
is empty. Hopefully, this makes the code a little bit simpler.

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/20230123205342.229589-12-martin@kaiser.cx


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