USB MIDI: Fix race condition that causes intermittent connections.

Plugging in a USB MIDI device normally works.
But occasionally the device is not seen due to a race condition.
This code retries until the device is ready.

Change-Id: Ifba9c9678e6be0c3d139f3467f895307096a2ad8
Signed-off-by: Phil Burk <philburk@google.com>