staging: r8188eu: xmit_buf's ff_hwaddr is not used

ff_hwaddr in struct xmit_buf is not used. Remove it.

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/20230207192319.294203-5-martin@kaiser.cx


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