staging: r8188eu: simplify frame type check

Reorder the code in rtw_make_wlanhdr to make the function simpler.
There's a large if statement to check that we process only data frames.
Revert the condition and exit for non-data frames.

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/20221230180646.91008-10-martin@kaiser.cx


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