xhci: Transition maintainership to Mathias Nyman.

I'm transitioning maintainership of the xHCI driver to my colleague,
Mathias Nyman.  The xHCI driver is in good shape, and it's time for me
to move on to the next shiny thing. :)

There's a few known outstanding bugs that we have plans for how to fix:

1. Clear Halt issue that means some USB scanners fail after one scan
2. TD fragment issue that means USB ethernet scatter-gather doesn't work
3. xHCI command queue issues that cause the driver to die when a USB
   device doesn't respond to a Set Address control transfer when another
   command is outstanding.
4. USB port power off for Haswell-ULT is a complete disaster.

Mathias is putting the finishing touches on a fix for #3, which will
make it much easier to craft a solution for #1.  Dan William has an
ACKed RFC for #4 that may land in 3.16, after much testing.  I'm working
with Mathias to come up with an architectural solution for #2.

I don't foresee very many big features coming down the pipe for USB
(which is part of the reason it's a good time to change now).  SSIC is
mostly a hardware-level change (perhaps with some PHY drivers needed),
USB 3.1 is again mostly a hardware-level change with some software
engineering to communicate the speed increase to the device drivers, add
new device descriptor parsing to lsusb, but definitely nothing as big as
USB 3.0 was.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@intel.com>