some bits for USB 2.0 playback support. I'm not 100% sure about

some bits for USB 2.0 playback support.  I'm not 100% sure about
scaling the frame rate to the polling interval, but it works on
the E-MU 0202 and it generally makes a lot of sense, especially
considering that this often brings the frames rate back to 1 kHz,
which is the frame rate of USB 1.x, which was the most current
USB spec when USB audio 1.0 spec was written.