hw/misc/max111x: Don't use vmstate_register()

hw/misc/max111x: Don't use vmstate_register()

The max111x is a proper qdev device; we can use dc->vmsd rather than
directly calling vmstate_register().

It's possible that this is a migration compat break, but the only
boards that use this device are the spitz-family ('akita', 'borzoi',
'spitz', 'terrier').

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200628142429.17111-8-peter.maydell@linaro.org