usb/dummy_hcd: don't probe for udc if hcd failed

the_controller is allocated in dummy_hcd_probe() and is NULL if the
allocation failed. The probe function of the udc driver is dereferencing
this pointer and fault.
Alan Stern suggested to abort the dummy_hcd driver probing so the module
is not loaded. The is abort-on-error has been also added to the udc
driver.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>