ipmi: Add alert handling to SSIF

The SSIF interface can optionally have an SMBus alert come in when
data is ready.  Unfortunately, the IPMI spec gives wiggle room to
the implementer to allow them to always have the alert enabled,
even if the driver doesn't enable it.  So implement alerts.
If you don't in this situation, the SMBus alert handling will
constantly complain.

Signed-off-by: Corey Minyard <cminyard@mvista.com>