mac-nubus-bridge: rename MacNubusState to MacNubusBridge

This better reflects that the mac-nubus-bridge device is derived from the
nubus-bridge device, and that the structure represents the state of the bridge
device and not the Nubus itself. Also update the comment in the file header to
reflect that mac-nubus-bridge is specific to the Macintosh.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210924073808.1041-15-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>