tty: goldfish: support platform_device with id -1

When the platform bus sets the platform_device id to -1 (PLATFORM_DEVID_NONE),
use an incrementing counter for the TTY index instead

Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Jin Qian <jinqian@android.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>