ipmi:ssif: Fix a memory leak when scanning for an adapter

The adapter scan ssif_info_find() sets info->adapter_name if the adapter
info came from SMBIOS, as it's not set in that case.  However, this
function can be called more than once, and it will leak the adapter name
if it had already been set.  So check for NULL before setting it.

Fixes: c4436c91 ("ipmi_ssif: avoid registering duplicate ssif interface")
Signed-off-by: Corey Minyard <minyard@acm.org>