hw/nvram: use at24 macro

Use the macro for going from I2CSlave to EEPROMState.

Signed-off-by: Patrick Venture <venture@google.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220119214329.2557049-1-venture@google.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>