usb: device: get rid of the partial initialization called by SYS_INIT

Part of the initialization is done in the static usb_device_init()
function called by SYS_INIT(). We can move part of it, which initialize
the descriptor and calls usb_set_config(), to usb_enable(). This allow
some usb_get_device_descriptor() dependencies called at application
desired point.

Also make sure that usb_fix_descriptor() is called only once.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>