x86/smm/smihandler.c: break case in switch

The case doesn't look like a deliberate fall-through,
since the next case (SNB/IVB/HSW) is more specific
than the one before it, so break out.

Change-Id: I55497aefe9e835842a82121270f2b2a9952f560d
Found-by: Coverity Scan
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/6571
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>