ipmi: ssif: Remove finished TODO comment about SMBus alert

commit 91620521 ("ipmi: Add alert handling to SSIF") introduces
support of SMBus alert. So, just remove TODO comment in order not to
confuse future readers.

Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
Message-Id: <20200624065405.17653-2-misono.tomohiro@jp.fujitsu.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>