usb-musb: Take a DeviceState* in init function

Initialise usb-musb by passing it a DeviceState* and the offset of the
IRQs in its gpio array, rather than a plain pointer to an irq array.
This is simpler for callers and also allows us to pass in a valid parent
to usb_bus_new(), so the USB bus actually appears in the qdev tree.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
