xhci: use trb helper functions when possible

Use the trb_is_link() and link_trb_toggles_cycle() helpers to
make the code more readable

No functional changes

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