machine/usb: move MIDI under usb/adc (Audio Device Class) package

Signed-off-by: deadprogram <ron@hybridgroup.com>

src/machine/usb/adc/doc.go

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+// package adc is for USB Audio Device Class devices.
+package adc

src/machine/usb/cdc/doc.go

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+// package cdc is for USB Communication Device Class devices.
+package cdc

src/machine/usb/descriptor/doc.go

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+// package descriptor is for the USB descriptor definitions.
+// For the actual implementations, see the individual packages.
+package descriptor

src/machine/usb/doc.go

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+// package usb contains the subpackages with USB descriptors and device
+// implementations for standard USB device classes such as the Communcation
+// Data Class (CDC), Human Interface Device (HID), and Audio Device Class (ADC).
+package usb

src/machine/usb/hid/doc.go

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+// package hid is for USB Human Interface Devices.
+package hid