The trb_in_td() function in the xHCI driver is supposed to translate a
physical transfer buffer request (TRB) into a virtual pointer to the ring
segment that TRB is in.

Unfortunately, a mistake in this function may cause endless loops as the
driver searches through the linked list of ring segments over and over
again.  Fix a couple bugs that may lead to loops or bad output:

1. Bail out if we get a NULL pointer when translating the segment's
private structure and the starting DMA address of the segment chunk.  If
this happens, we've been handed a starting TRB pointer from a different
ring.

2. Make sure the function works when there's multiple segments in the
ring.  In the while loop to search through the ring segments, use the
current segment variable (cur_seg), rather than the starting segment
variable (start_seg) that is passed in.

3. Stop searching the ring if we've run through all the segments in the
ring.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>