xhci: Clean up cycle bit math used during stalls.

Use XOR to invert the cycle bit, instead of a more complicated
calculation.  Eliminate a check for the link TRB type in find_trb_seg().
We know that there will always be a link TRB at the end of a segment, so
xhci_segment->trbs[TRBS_PER_SEGMENT - 1] will always have a link TRB type.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Tested-by: Takashi Iwai <tiwai@suse.de>