staging: r8188eu: make xmitframe_swencrypt a void function

xmitframe_swencrypt always returns _SUCCESS and the caller does not check
the return value. We can remove the return value and make
xmitframe_swencrypt a void function.

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/20221230180646.91008-2-martin@kaiser.cx


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