xhci: Warn when hosts don't halt.

Eric Fu reports a problem with his VIA host controller fetching a zeroed
event ring pointer on resume from suspend.  The host should have been
halted, but we can't be sure because that code ignores the return value
from xhci_halt().  Print a warning when the host controller refuses to
halt within XHCI_MAX_HALT_USEC (currently 16 seconds).

(Update: it turns out that the VIA host controller is reporting a halted
state when it fetches the zeroed event ring pointer.  However, we still
need this warning for other host controllers.)
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>

drivers/usb/host/xhci.c

@@ -106,6 +106,9 @@ int xhci_halt(struct xhci_hcd *xhci)
 			STS_HALT, STS_HALT, XHCI_MAX_HALT_USEC);
 	if (!ret)
 		xhci->xhc_state |= XHCI_STATE_HALTED;
+	else
+		xhci_warn(xhci, "Host not halted after %u microseconds.\n",
+				XHCI_MAX_HALT_USEC);
 	return ret;
 }