staging: r8188eu: return immediately if we're not meant to encrypt

In function xmitframe_swencrypt, we can return immediately if our packet
needs no encryption. This is simpler than wrapping all the code into a
large if statement.

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/20221230180646.91008-5-martin@kaiser.cx


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