xhci: Avoid infinite loop when sg urb requires too many trbs

Currently prepare_ring() returns -ENOMEM if the urb won't fit into a
single ring segment.  usb_sg_wait() treats this error as a temporary
condition and will keep retrying until something else goes wrong.

The number of retries should be limited in usb_sg_wait(), but also
prepare_ring() should not return an error code that suggests it might
be worth retrying.  Change it to -EINVAL.

Reported-by: jidanni@jidanni.org
References: http://bugs.debian.org/733907
Fixes: 35773dac5f86 ('usb: xhci: Link TRB must not occur within a USB payload burst')
Cc: stable <stable@vger.kernel.org> # 3.12
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>

drivers/usb/host/xhci-ring.c

@@ -3000,7 +3000,7 @@ static int prepare_ring(struct xhci_hcd *xhci, struct xhci_ring *ep_ring,
 			if (num_trbs >= TRBS_PER_SEGMENT) {
 				xhci_err(xhci, "Too many fragments %d, max %d\n",
 						num_trbs, TRBS_PER_SEGMENT - 1);
-				return -ENOMEM;
+				return -EINVAL;
 			}
 
 			nop_cmd = cpu_to_le32(TRB_TYPE(TRB_TR_NOOP) |