arch/arm/src/stm32/stm32_usbdev.c: On a failure to bind the class driver, the...

arch/arm/src/stm32/stm32_usbdev.c: On a failure to bind the class driver, the driver reference was being nullified too soon.  This caused an exception in usbdev_reset() later.  The driver reference will be null
ified later usbdev_unregister when the caller gets the error.  From David Sidrane