ipmi_si: Fix error handling of platform device

Cleanup of platform devices created by the IPMI driver was not
being done correctly and could result in a memory leak.  So
create a local boolean to know how to clean up those platform
devices.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>