xhci: Fix error path return value.

This patch fixes an issue discovered by Dan Carpenter:

The patch 3b3db026: "xhci: Add infrastructure for host-specific
LPM policies." from May 9, 2012, leads to the following warning:
drivers/usb/host/xhci.c:3909 xhci_get_timeout_no_hub_lpm()
         warn: signedness bug returning '-22'

  3906          default:
  3907                  dev_warn(&udev->dev, "%s: Can't get timeout for non-U1 or U2 state.\n",
  3908                                  __func__);
  3909                  return -EINVAL;
                        ^^^^^^^^^^^^^^
This should be a u16 like USB3_LPM_DISABLED or something.

  3910          }
  3911
  3912          if (sel <= max_sel_pel && pel <= max_sel_pel)
  3913                  return USB3_LPM_DEVICE_INITIATED;

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>