After playing tune on Phasor, get a bit of noise when testing Phasor

[tomcw]

Repro:

• mb-audit v1.1 (preview 0.2)
• do 'Music Test' for a few seconds (perhaps until it uses the AY noise channel?)
• ESC to exit, then do 'Audit & Diagnostics'
• the MB-C (s4) makes no extra noise
• the Phasor (s1) make ~1s of noise before saying 'Classic Adventure'
• NB. doesn't happen everytime

[tomcw]

Still happens at 18e9fc815, and during soak testing at component = 0x10, ie. during Check6522(). But noise stops after DetectSubunits().

Makes sense, since DetectSubunits() calls DetectAY8913(), which does Init6522ForAY(), resulting in a AY RESET for both the AYs attached to that 6522.

[tomcw]

Still happens at 1.53