char: ipmi: eliminate misleading print info when being probed via ACPI

When ipmi is probed via ACPI, the boot log shows

[   17.945139] ipmi_si IPI0001:00: probing via device tree
[   17.950369] ipmi_si IPI0001:00: ipmi_si: probing via ACPI
[   17.955795] ipmi_si IPI0001:00: [io  0x00e4-0x3fff] regsize 1 spacing 1 irq 0
[   17.962932] ipmi_si: Adding ACPI-specified bt state machine

which "ipmi_si IPI0001:00: probing via device tree" is misleading
with a ACPI HID "IPI0001" but probing via DT.

Eliminate this misleading print info by checking of_node is valid
or not before calling of_ipmi_probe().

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>