ipmi:bt-bmc: Avoid unnecessary check

bt_bmc_probe() is only called with an openfirmware platform device.
Therefore there is no need to check that the passed in device is NULL or
that it has an openfirmware node.

Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Message-Id: <20200408115958.2848-1-tangbin@cmss.chinamobile.com>
[Fixed the title up a bit.]
Signed-off-by: Corey Minyard <cminyard@mvista.com>