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usbehci: clean cache unconditionally before handing a buffer to the hardware

even in the read case, we need to clean the cache
so the cpu will not flush out old changes while
the hardware updates the buffer.

--- a/sys/src/9/port/usbehci.c

+++ b/sys/src/9/port/usbehci.c

@@ -2171,11 +2171,10 @@

 		td->buff = (*ctlr->dmaalloc)(count+0x1000);

 		td->data = (uchar*)ROUND((uintptr)td->buff, 0x1000);

 	}

-	if(a != nil && count > 0){

+	if(a != nil && count > 0)

 		memmove(td->data, a, count);

-		if(ctlr->dmaflush != nil && td->buff != nil)

-			(*ctlr->dmaflush)(1, td->data, count);

-	}

+	if(ctlr->dmaflush != nil && td->buff != nil)

+		(*ctlr->dmaflush)(1, td->data, count);

 	pa = PADDR(td->data);

 	for(i = 0; i < nelem(td->buffer); i++){

 		td->buffer[i] = pa;