usb: add a HCD_DMA flag instead of guestimating DMA capabilities

The usb core is the only major place in the kernel that checks for
a non-NULL device dma_mask to see if a device is DMA capable.  This
is generally a bad idea, as all major busses always set up a DMA mask,
even if the device is not DMA capable - in fact bus layers like PCI
can't even know if a device is DMA capable at enumeration time.  This
leads to lots of workaround in HCD drivers, and also prevented us from
setting up a DMA mask for platform devices by default last time we
tried.

Replace this guess with an explicit HCD_DMA that is set by drivers that
appear to have DMA support.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20190816062435.881-4-hch@lst.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/uhci-grlib.c

@@ -63,7 +63,7 @@ static const struct hc_driver uhci_grlib_hc_driver = {
 
 	/* Generic hardware linkage */
 	.irq =			uhci_irq,
-	.flags =		HCD_MEMORY | HCD_USB11,
+	.flags =		HCD_MEMORY | HCD_DMA | HCD_USB11,
 
 	/* Basic lifecycle operations */
 	.reset =		uhci_grlib_init,