Fix a few bugs when gpiobus is detaching:

• Detach the gpiobus and the gpioc devices from the GPIO controller.
• Fix the leak of gpiobus IRQ rman(9) region descriptor.
• Fix the leak of child ivars and IRQ resource list.

While here return NULL (instead of 0) for a device_t that fails to allocate
the ivar memory.

Tested with gpiobus built as a module.

Sponsored by: Rubicon Communications (Netgate)