mboxd: Update udev rule for /dev/aspeed-mbox

At least one bug prevented the discovery that the udev rule was broken,
mainly that the Wants/After directives were in the wrong section of the
systemd unit file[1]. systemd logged and ignored the broken Wants/After
directives and launched the service anyway, which lead to the
impression that everthing was working as expected.

Fix the problem by inspecting the device with udevdm to determine it's
properties:

    $ udevadm info -q all -a /dev/aspeed-mbox`:

      looking at device '/devices/platform/ahb/1e789000.lpc/1e789080.lpc-host/1e789200.mbox/misc/aspeed-mbox':
        KERNEL=="aspeed-mbox"
        SUBSYSTEM=="misc"
        DRIVER==""
    ...

[1] https://github.com/openbmc/openbmc/issues/999



Change-Id: I91c410da44224ee7b5c527ee4a744b7e7ecd5a9b
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>