firmware: google: Release devices before unregistering the bus

Fix a bug where the kernel module can't be loaded after it has been
unloaded as the devices are still present and conflicting with the
to be created coreboot devices.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Link: https://lore.kernel.org/r/20191118101934.22526-2-patrick.rudolph@9elements.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>