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

drivers/staging/r8188eu/include/rtw_xmit.h

@@ -193,7 +193,6 @@ struct xmit_buf {
 	u32 alloc_sz;
 	u32  len;
 	struct submit_ctx *sctx;
-	u32	ff_hwaddr;
 	struct urb *pxmit_urb;
 	int last[8];
 };