USB: dwc2: hande irq on dead controller correctly

If the controller is dead, the honest answer to the
question whether it has caused an irq is: unknown
As the purpose of the irq return is to trigger switching
off an IRQ, the correct response if you cannot
determine if your device has caused the interrupt is
IRQ_HANDLED

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Link: https://lore.kernel.org/r/20230822112455.18957-1-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>