usb - Update bus/u4b

usb - Update bus/u4b

* Update bus/u4b from FreeBSD to commit 3121e258c76aa, 10 March 2015,
  with the following commit message:

      Lock softc before clearing bits.

* Some bits not updated.  Some changes around the MSI handling work
  differently in DFly so I punted on that.  And the serial/tty in FreeBSD
  is a bit different, particular this 'pps' stuff.

* Numerous bits of code currently conditionalized out use ABI features
  from FreeBSD, particularly RWTUN, which we do not yet have.  Currently
  non-critical, we can fix these as the related code gets used (if the
  related code gets used).

Reviewed-by:  Markus Pfeiffer