ipmi: change addr_source to an enum rather than strings

Switch from a char* to an enum to identify the address source of SIs,
making it easier to handle them appropriately during registration.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>