[PATCH] IPMI: system interface hotplug

Add the ability to hot add and remove interfaces in the ipmi_si driver.  Any
users who have the device open will get errors if they try to send a message.

Signed-off-by: Corey Minyard <minyard@acm.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>