xhci: Report max device limit when Enable Slot command fails.

xHCI host controllers may only support a limited number of device slot
IDs, which is usually far less than the theoretical maximum number of
devices (255) that the USB specifications advertise.  This is
frustrating to consumers that expect to be able to plug in a large
number of devices.

Add a print statement when the Enable Slot command fails to show how
many devices the host supports.  We can't change hardware manufacturer's
design decisions, but hopefully we can save customers a little bit of
time trying to debug why their host mysteriously fails when too many
devices are plugged in.
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Reported-by: Amund Hov <Amund.Hov@silabs.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci.c

@@ -3696,6 +3696,9 @@ int xhci_alloc_dev(struct usb_hcd *hcd, struct usb_device *udev)
 
 	if (!xhci->slot_id) {
 		xhci_err(xhci, "Error while assigning device slot ID\n");
+		xhci_err(xhci, "Max number of devices this xHCI host supports is %u.\n",
+				HCS_MAX_SLOTS(
+					readl(&xhci->cap_regs->hcs_params1)));
 		return 0;
 	}