staging: r8188eu: simplify rtw_alloc_xmitframe

Make the rtw_alloc_xmitframe function a bit simpler.

The container_of() call never returns NULL. The if (pxframe) check is
false only if pfree_xmit_queue is empty. Handle this special case
explicitly and save one level of indentation.

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/20230207192319.294203-3-martin@kaiser.cx


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