DA146xx are Bluetooth devices that may be battery
powered and when not connected to USB host there
is no need for USB peripheral to be running.
This change allows to enable USB peripheral when
VBUS is present otherwise USB is down reducing
power consumption.
tud_vsub_changed() function must be called
whenever VBUS change was detected.
For bus-powered devices this function should be called
at startup since VBUS must be present while device
is working.
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