staging: r8188eu: merge dequeue_one_xmitframe into its caller

dequeue_one_xmitframe is a small function that is called only from
rtw_dequeue_xframe. Merge the two functions.

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/20230123205342.229589-18-martin@kaiser.cx


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