xhci: rework inc_deq() and fix off by one error.

inc_deq() is called both for rings with link trbs and the event ring
without link trbs.
The last_trb() check in inc_deq() has a off by one error, going beyond
allocated array when checking if trb == [TRBS_PER_SEGMENT], and the whole
inc_deq() depend on this.

Rewrite the inc_deq() funciton, remove the faulty last_trb() helper, add
new last_trb_on_seg() and last_trb_on_ring() helpers

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