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/core/hcd.c

@@ -2454,7 +2454,6 @@ struct usb_hcd *__usb_create_hcd(const struct hc_driver *driver,
 	hcd->self.controller = dev;
 	hcd->self.sysdev = sysdev;
 	hcd->self.bus_name = bus_name;
-	hcd->self.uses_dma = (sysdev->dma_mask != NULL);
 
 	timer_setup(&hcd->rh_timer, rh_timer_func, 0);
 #ifdef CONFIG_PM