usb: xhci: fix link trb decoding

xhci_decode_trb() treats a link trb in the same way as that for
an event trb. This patch fixes this by decoding the link trb
according to the spec.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>