USB MIDI: Reset counter after unique codes full

Some tests continously plug and unplug MIDI devices.
After 1000 operations, there is an infinite while loop.

This CL clears up the hashset if it is nearly full.

Bug: 234688233
Test: Plug and unplug MIDI devices
Change-Id: Ie0c6359c07694e3f6cc4a3011fd39a525314ab94