usb: device_next: Update IAD first interface on init

IAD must be before the interfaces it associates and therefore there is
no need for the class to be in charge of updating the bFirstInterface.
Update IAD in common initialization code and remove the updates from
classes.

This fixes UAC2 instances where the IAD bFirstInterface is not 0, e.g.
when HID was used together with UAC2.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>