usb: musb_am335x: make module unload/reload work

usb: musb_am335x: make module unload/reload work

Currently unloading and reloading the usb-am335x module will render
the USB driver non-functional. This is because of_platform_populate()
does nothing upon reload because the OF_POPULATED flag will still be
set on the device nodes from the first module load.

Clearing this flag when unregistering the devices makes this driver
functional also upon repeated unload/load sequences.

Conflicts:
	drivers/usb/musb/musb_am335x.c