xhci: Bigendian fix for skip_isoc_td()

Commit 926008c9 "USB: xhci: simplify logic
of skipping missed isoc TDs" added a small endian bug.  This patch
fixes skip_isoc_td() to read the DMA pointer correctly.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>